You just have an older build of Flash Builder... the stock location of playerglobal.swc changed in later SDK builds. When a public update to Flash Builder is released it will be in synch with the later version of the SDK you are using.
Full scoop here with workaround: http://forums.adobe.com/message/2508943#2508943.
Thanks CoreyRLucier. Yeah...for others that may have a hard time with that post, I did the following:
In FlashBuilder, right-click on your Flex project in the Package Explorer and navigate to: Properties > Flex Build Path > Library path.
In the Flex 4.0 (build XXX), remove the playerglobal.swc and 10.minorplayerversion swc. Then click on your lib folder from the list and add playerglobal.swc by clicking on the 'Add SWC...' button. The path to my playerglobal.swc was the following:
/Applications/Adobe Flash Builder Beta 2/sdks/18.104.22.16818/frameworks/libs/player/10.0/playerglobal.swc
See picture if needed.