There is a right and a wrong way to set up Flex Builder. The wrong way is to get the Flex Builder package from the Adobe site. It’s running on a super old version of Eclipse, and it lacks a lot of useful editors and functionality. I have lost many man-hours of productivity to this version of Flex, which is why I am writing this post.
What you want to do is this: Go get the latest, vanilla Java-IDE version of Eclipse. You can do this at the Eclipse downloads page, the package called “Eclipse IDE For Java Developers.” (Not Java SE, unless you want an extra hundred megs of Java tools.)
Now, go to the Adobe Flex download page, and go to download it – but scroll waaay down and get the plug-in instead.
Finally, install Eclipse – then install the plugin. Now you have a fully up to date, ready to go version of Flex, as well as a bunch of nice tools that come with Eclipse for Java. In my experience, this is significantly more reliable and responsive than the all-in-one build from Adobe.
Edit: Don’t forget to do Help -> Search for Flex Builder Updates and Help -> Software Updates… to make sure that you are fully up to date!