If you have been working with Eclipse 3.2 or 3.3 with flex builder 3, and if the upgrade to 3.4 broke the builder, try the following patch http://www.tekool.net/blog/2008/06/28/flex-builder-3-plugin-dont-work-with-eclipse-34/
Just download the patch from here unzip it somewhere on your desktop and copy the plugins content ( 2 jar files) to your eclipse’s plugins folder. If you have any existing files while copying good idea would be to take a backup before doing so.
Now restart eclipse with -clean option. Your flex builder should work as it used to before.