If there are none listed you just click add new and browse to the correct folder containing the SDK.See bellow for the default paths on Windows and the Mac OS.If you want to do the upgrade to Eclipse 3.5 (which I do recommend) Ill give you a bit of a warning about the bugs I have found so far using Eclipse 3.5 with Flex Builder 3 after the jump.
You'll get the lovely generic "An internal build error has occurred right click for more information".
In a previous entry, “Downloading and installing Flex SDK builds from opensource.adobe.com”, we saw how to download and install nightly builds of the Flex 3 SDK into Flex Builder 3.
Then just simply copy the file called com.adobe.flexbuilder.dolphin from your Eclipse 3.4 installs dolphin folder into your new Eclipse 3.5 installs dolphins folder.
Then just launch Eclipse with the --clean option and you should be able to use Flex Builder 3 in Eclipse 3.5.
You may have to hit refresh on the design view to get rid of the error message that may or may not appear.
This issue is probably caused by the configuration folder not containing information about the plugin.
All and all taking the time to install the Flex Builder 3 plug-in into Eclipse 3.5 is worth the time and the odd inconveniencing bug for the many improvements (and the more stable) in Eclipse 3.5.
Oh and one last thing the Flex Builder 3 plug-in installer probably wont work so install it into a install of Eclipse 3.4.
UPDATE So I recently started working on a project (my first from scratch sense moving to Eclipse 3.5, and I've noticed another bug.
This time its a bit of a show stopper, first lets start with the creating a project.
You will notice that your projects compile like normal and run like normal.