This is happening to me too.  I'm a CS Extension Builder n00b.  I just downloaded 2.0 today, and created an extension targeting Flash Pro CS6, using Flash Builder 4.6.  I get lots of these warnings:

[INFO] CSAWLib.CSLogger TypeConUtils.getAnyPossibleClassDefinitionFor() Failed to find any class definition for com.adobe.flashpro::Array

But eventually, I get a similar error that prevents any further progress:

[ERROR] CSAWLib.CSLogger CSHostObject.hostGet() failed on elements -  with msg: Failed to find any class definition for com.adobe.flashpro::SymbolInstance

I was just exploring down into a MovieClip timeline.  I haven't changed any project settings since it was setup.  Here's the code:

package 
{
     import com.adobe.csawlib.flashpro.FlashPro;
     import com.adobe.flashpro.*;
     
     public class ExportFlashPro
     {
          public static function run():void 
          {
               var app:Flash = FlashPro.app;
               var doc:Document = app.getDocumentDOM();
               if( doc == null )
               {
                    app.outputPanel.trace( "No document opened." );
                    return;
               }
               
               var timeline:Timeline = doc.getTimeline();
               for( var f:int=0; f=timeline.frameCount; ++f )
               {
                    for( var l:int=timeline.layerCount-1; l>=0; --l )   // Loop backwards, back to front.
                    {
                         var layer:Layer = timeline.layers[ l ] as Layer;
                         if( layer.layerType != "normal" )
                              continue;
                         if( layer.frameCount < f )
                              continue;     // This layer doesn't go that far.
                         
                         var frame:Frame = layer.frames[ f ] as Frame;
                         if( frame.elements )
                         {
                              var elementCount:int = frame.elements.length;
                              for( var e:int=elementCount-1; e>=0; --e )     // Loop backwards, back to front.
                              {
                                   var element:Element = frame.elements[ e ];
                              }
                         }
                    }
               }               
          }
     }
}