1. Launch Flex Builder 3
2. Go to Help > Software Updates > Find and Install
3. Search for new features to install, click next
4. Select “The Eclipse Project Updates”, click finish
   Note: If you do not have the option above click New Remote Site and enter "The Eclipse Project Updates" as the name and "http://update.eclipse.org/updates/3.3" as the url.
5. In Eclipse Project Updates > Eclipse SDK Eclipse 3.3.1 Select “Eclipse Java Development Tools”, click next
6. Accept the license agreement, click next
7. Click finish to start download
8. Eclipse downloads Java Development Tools
9. Click “Install all” to install Java Development Tools
10. Restart the Eclipse workbench

FROM: http://www.judahfrangipane.com/blog/?p=193