Take a look in this directory:
C:\Documents and Settings\<Your User Name>\Application Data\Adobe\Dreamweaver CS4\en_US\Configuration\Shared\WebAssist\DynamicDropDowns\Library\Shared_Ser.js
One other thing you can try that should be quick is close Dreamweaver and Extension manager and delete your MacFileCache file in the Configuration directory and then open up Dreamweaver and try the extension again:
C:\Documents and Settings\<Your User Name>\Application Data\Adobe\Dreamweaver CS4\en_US\Configuration\
