I couldn't get this working for a while. I had a stock eclipse setup from the Ubuntu repository that I had put Flex Builder in. Next went to install PDT but couldn't get it working whether through a repository, through the eclipse update site, etc.

I didn't realize you need to run eclipse as root to install plugins. and it provides no error message telling you the installation was a complete failure- it just fails to write the plugin files silently. doh!

Here's how to get it working:

- start eclipse as root - sudo eclipse
- Help > Install new software