How to get Eclipse 3.5 (Galileo) and Flex Builder working together.
Have you downloaded the latest version of Eclipse and you can’t get it to work with the Adobe Flex Builder plug-in? Well, here are a few steps you can take to hopefully get it to work again.
First, download and install Eclipse 3.5.
Second, if you do not already have Flex Builder installed, download and install as well. Once the installation is complete, navigate to where you installed Eclipse, for example: C:/Program Files/Eclipse. This should be the directory or folder that contains the eclipse.exe file or executable. In this directory, make sure another folder called ‘links’ exists. If it does not, create it.
Enter the ‘links’ directory and create a new text file called ‘com.adobe.flexbuilder.feature.core.link’ without any closing file extension such as .txt. Edit this file and add the following contents into it:
path=C:/Program Files/Adobe/Flex Builder 3 Plug-in
Replace ‘C:/Program Files/Adobe/Flex Builder 3 Plug-in’ with the path to your Flex Builder installation directory or folder.
Save the file and close it. Then launch Eclipse. Now go to Window Menu, select “Open Perspective” then “Other.” You should see your Flex Development Perspective listed. If you previously entered a license key for Flex Builder, it will recognize it for you. Otherwise, it will ask you to enter your license key.
Voila! You should have a working Eclipse 3.5/Flex Builder environment.
Credit for this find goes to Adam Ness.
Robert Frew's Blog 
This solution work for me only for work on flex proyect. The problem is don’t compile and the error is “internal build error has occurred”, no more info. I probe many options but don’t work.
Didn’t work for me.
Thank You Soooooooo Muchhhhh….
If the flex builder plug-in is Windows®, English | 338.8 MB,
and the Eclipse is eclipse-jee-galileo-SR2-win32,
your solution work!
else makesure you downloaded the latest version of Eclipse and flex builder plug-in.
Thank you so much!