try closing Dreamweaver, then going to the following location based on your OS:
Win XP:
C:\Documents and Settings\<Your User Name>\Application Data\Adobe\Dreamweaver CS4\en_US\Configuration\Shared\WebAssist\Memento\Notifications
Win Vista
C:\Users\<Your User Name>\AppData\Roaming\Adobe\Dreamweaver CS4\en_US\Configuration\Shared\WebAssist\Memento\Notifications
delete all of the files in the Notifications folder, then restart Dreamweaver.