Sounds like the WebAssist installer did not run.
After installing the WebAssist extensions in the DW Extension manager, when you open DW, it should launch the WebAssist installer. if the WebAssist installer does not open automatically, open a php page to force it to start. After the WebAssist installer completes, make sure to restart DW.
you will also need to install the Deprecated server behaviors extension availbale from Adobe:
Also, make sure you have your site defined for dynamic development, see the following tutorial:
and that you have a database connection set up:
Note that you can only add CMS content to a php page, If you are working on a static html page, it wont be able to work.
if you continue to have problems, please include the text of the error message.