I had to uninstall and reinstall Flash Builder due to several crash's. After I re-installed I wanted to make sure Flex was up to date, went to Help > Software Update. Now I get the prompt
"No Updates Found - There are no update sites to search. Do you wish to open the 'Available Software Sites' preferences?"
See http://blogs.adobe.com/jasonsj/2010/03/installing_plug-ins.html. Assuming you're using FB 4 or 4.0.1, use http://download.eclipse.org/releases/galileo/. If you're using the FB Burrito preview release, use http://download.eclipse.org/releases/helios.
Jason San Jose
Software Engineer, Flex Mobile