it's probably not the correct way to do it but I am more familiar with Flex4 (which use states) but this works

however i had to remove your Alert they were throwing an error

<mx:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
                                        xmlns:mx="library://ns.adobe.com/flex/mx"
                                        xmlns:s="library://ns.adobe.com/flex/spark">
 
 
          <fx:Script>
                    <![CDATA[
                              import mx.controls.Alert;
                              public var u:String;
                              public var p:String;
  
                              public function log():void
                              {
                                        u = user.text.toLowerCase();
                                        p = pass.text.toLowerCase();
                                        if ((u == '' || u != 'webmultimedia') || (p == '' || p != 'abc'))
                                        {
                                                  //Alert.show('Wrong userid or Password', 'Sorry', Alert.OK | Alert.CANCEL);
                                        }
                                        else
                                        {
                                                  //Alert.show('Hi', 'Welcome', mx.controls.Alert.OK);
                                                  stockMarket.visible = true;
                                                  NewForm.visible = false;
                                        }
                              }
  
                              public function clear():void
                              {
                                        user.text = '';
                                        pass.text = '';
  
                              }
                    ]]>
          </fx:Script>
 
 
 
 
          <mx:Panel id="NewForm"
                                title="Please Log In"
                                height="214"
                                width="285"
                                horizontalAlign="center"
                                verticalAlign="middle"
                                horizontalCenter="23"
                                verticalCenter="2">
                    <mx:Label id="UserName"
                                          text="User Name" />
                    <mx:TextInput id="user" />
                    <mx:Label id="password"
                                          text="Password" />
                    <mx:TextInput id="pass"
                                                    displayAsPassword="true" />
                    <mx:Button id="submit"
                                           label="Submit"
                                           click="log()" />
          </mx:Panel>
 
 
 
 
          <mx:Panel id="stockMarket"
                                visible="false"
                                width="349"
                                height="383"
                                chromeColor="#1B8905"
                                color="#FFFFFF"
                                focusColor="#0F3CFF"
                                fontFamily="Georgia"
                                fontSize="21"
                                fontWeight="bold"
                                paddingBottom="10"
                                paddingLeft="10"
                                paddingRight="10"
                                paddingTop="10"
                                title="Stock Market">
 
 
                    <!--First Pane Of TabNavigator-->
                    <mx:TabNavigator borderStyle="solid">
                              <mx:VBox y="71"
                                                   width="371"
                                                   height="334"
                                                   label="Dow Jones"
                                                   chromeColor="#0A741C">
                                        <mx:Image id="loader1"
                                                              source="http://dl.dropbox.com/u/8861424/stock1.png" />
 
 
                                        <mx:RichTextEditor id="myRTE"
                                                                                   width="355"
                                                                                   height="286"
                                                                                   borderVisible="true"
                                                                                   color="#000000"
                                                                                   text="Enter Stock Comment" />
 
 
                              </mx:VBox>
 
 
                              <!-- Second pane of TabNavigator -->
                              <mx:VBox label="SandP"
                                                   width="500"
                                                   height="350">
                                        <mx:Image id="loader2"
                                                              source="http://dl.dropbox.com/u/8861424/Untitled.png" />
 
 
 
 
                                        <mx:RichTextEditor id="myRTE2"
                                                                                   width="355"
                                                                                   height="286"
                                                                                   borderVisible="true"
                                                                                   color="#000000"
                                                                                   text="Enter Stock Comment" />
 
 
                              </mx:VBox>
 
 
                              <!-- Second pane of TabNavigator -->
                              <mx:VBox label="Nasdaq"
                                                   width="500"
                                                   height="350">
                                        <mx:Image id="loader3"
                                                              source="http://dl.dropbox.com/u/8861424/stock2.png" />
 
 
                                        <mx:RichTextEditor id="myRTE3"
                                                                                   width="355"
                                                                                   height="286"
                                                                                   borderVisible="true"
                                                                                   color="#000000"
                                                                                   text="Enter Stock Comment" />
                              </mx:VBox>
 
 
                    </mx:TabNavigator>
          </mx:Panel>
</mx:Application>