You should be using free Tools version 1.0.9 in Dw CS6, make sure you have the correct version. If you have a previous version you will need to uninstall it from the extension manager and install the most recent.
after installing the extension, you will need to open DW, then open a php page to start the webassist installer, after the web assist installer completes, make sure to restart DW before trying to use it.
also, reset the workspace layout:
go to Window -> Work Space layout -> Reset '<your layout name>'
and Delete the Dreamweaver Cache file from the users configuration folder:
I am using the latest vesion (according to your website) and I have now done all that you suggested. The serial number is not being accepted as valid. Seriously ...