Is this a clean install of Site import? Or did you have a previous version installed before and updated it?
If this happened after updating, did you remover the extension form the extension manager before installing the update?
Your configuration folder may have become corrupted. You can rename the configuration folder to have Dreamweaver recreate it. Close Dreamweaver and go to:
C:\Documents and Settings\<Your User Name>\Application Data\Adobe\Dreamweaver CS4\en_US
(Please note that <Your User Name> Should be replaced with the User name you use to log into your computer)
Rename the configuration folder to configuration_back. This will have the effect of disabling all of the installed extensions. You should open the extension manager and remove all extensions to clear the extension mangers cache. After removing the extensions you can then reinstall them making sure to download the latest versions from your order history.
On Windows Computers, Application Data folders may be set to be hidden.
To View Hidden Folders in Windows XP:
1) On the Tools menu in Windows Explorer, click Folder Options.
2)Click the View tab.
3) Under Hidden files and folders, click Show hidden files and folders.