You and I have basically the same setup, so I can confirm that CSS Form Builder works.
What you may want to try - as this is what I have done when new extensions have caused problems - is to delete the MacFileCache file.
It is located in (username)/Library/Application Support/Adobe/Dreamweaver CS4/en_US/Configuration/
It will look like this: MacFileCache-A58CCE96.dat (of course the letters and numbers are arbitrary based on the system. I would delete the file and then restart Dreamweaver CS4 with the new extension installed.