A few days ago I had installed the Flex Builder Plugins on a staging eclipse ganymede installation where I typically would test plugins. The installer created a link /links/com.adobe.flexbuilder.feature.core.link with the content

d:/Java/Flex Builder 3 Plug-in

To activate this plugins in my productive eclipse installation I just copied this link file and restarted eclipse but the flex plugins didn’t get activated. After a while I found a discussion in some Adobe forum.

So I changed the contents of the link file to:

path=d:/Java/Flex Builder 3 Plug-in

Now it works! Unfortunately the eclipse documentation on this also didn’t clarify that.