Stage 3D is only available in the Flash Player 11 beta at the moment and there a few steps to get it working correctly. A few of the last ones tripped me up so I thought I'd list them here for anyone else who is having trouble.
- Download and install Flash Player 11 beta
- Download at least Flex 4.5 and unpack anwhere
- Install Flash Player 11 playerglobal.swc:
- Download playerglobal.swc (bottom of page)
- Rename downloaded swc to 'playerglobal.swc'
- Place playerglobal.swc into Flex SDK folder under /frameworks/libs/player/11.0
- Install Flex SDK in Preferences -> Flash Builder -> Installed Flex SDKS -> navigate to adobe Flex SDK
- Create a web based Actionscript or Flex Project in Flash Builder with the above Flex SDK
- Go into the project properties -> compiler settings
- Set User specific version to 11.0.0
- Add compiler argument -swf-version=13
- Make sure generate HTML wrapper file is checked
- Open html-template/index.template.html and add params.wmode = "direct"; at line 49
- Make sure your video card drivers are up to date and are newer than 2009
- Lastly right click on any flash content and go to settings and make sure 'Enable hardware acceleration' is ticked.
All those steps except the last 3 are required to compile with Flash Player 11. Setting the wmode to direct, having up to date video card drivers and making sure hardware acceleration is enabled are required for Stage 3D to work.
Hope this helps.