First I install flex (build: 3.0.194161) to use with Eclipse 3.2.1, it’s working fine.

I wan to use my existing flex plugin with Eclipse 3.4, I got problem as describe in this post.

I wanna to try to use both eclipse with the same flex plugin, with Eclipse 3.4 I try to install Flex plugin by Software Updates but I could not.

I found the solution on the net, thanks to

I have removed the Flex plugin and then re-install the flex plugin, point to Eclipse 3.4 and Proceed it with Caution

After done, download the patch provided in (Direct download here)
Replace the plug-in of the patch into Eclipse plug-in of Flex Plug-in

And at Eclipse start up add parameter -clean: eclipse.exe -clean
Or from the Flex launcher add this parameter too:
eclipse.exe -clean -showlocation -vm …

It’s working for me now.