I have had this issue multiple times and feel I have discovered the best way to fix it...
Please follow the below steps - and let me know if your happy with the results. Please read the whole post before continuing.
1. Close Dreamweaver
2. Navigate to inside Configuration Folder - C:\Documents and Settings\pc\Application Data\Adobe\Dreamweaver CS4\en_US\Configuration ( This is my path, yours may be different )
3. Delete the Cache File
4. Restart PC
5. Navigate to the config folder C:\Documents and Settings\pc\Application Data\Adobe\Dreamweaver CS4\en_US ( again your path may be different ). You should see only one folder called "Configuration"
6. Rename this folder to "Configuration_old"
7. Start Dreamweaver
8. Goto Commands -> Manage Extensions
9. At this point you will see all of your Extensions but they are inactive. Click on Data Assist ( or whatever extension you wish to use ). Dreamweaver will then activate/re-install the extension.
10. Close and restart Dreamweaver
11. Try the Data Assist extension ( you will need to re-enter your Serial to Activate )
Note - this de-activates all of your current extensions and you will need to re-enter serial numbers to re-activate, so if you have any other extensions that are not from Web Assisit please make sure you have your serial number to re-activate.
At the end of the day this issue should not happen - but the amount of time Web Assist Extensions have saved me in development, Im more than happy to deal with a few quirks here and there.