What Version of Site Assist do you have installed? 1.0.7 is the most recent version of Site Assist professional, if you have a previous version, uninstall it from the extesnion manager, and install version 1.0.7.
What is your OS?
What version of Dreamweaver?
make sure the Disable Preview option is not set. Got to Edit -> WebAssist Preferences.
in the preview section, toggle on then off the Disable preview checkbox then click OK.