The error suggests that the WebAssist installer did not run.
Close DW, then Delete the Dreamweaver Cache file from the users configuration folder:
next open the extension manger, and uninstall Data Bridge, then install it again.
when you open DW, it should launch the web assist installer, if the WA Installer does not launch automatically, open a php page to force it to start.
After the WA Installer completes, make sure to restart DW.