Sorry, I should've explained what that would do more clearly. Deleting the configuration folder for the extension manager has the effect of removing all the extensions that are installed. Try deleting that configuration and also the Dreamweaver configuration folder at the same time. the Dreamweaver folder is located in the same place the Extension Manager one is and it should be the Dreamweaver 9 folder.
Deleting the Dreamweaver 9 configuration folder has the effect of disabling all the extensions installed.
After doing both of these actions, you shouldn't have any conflicts when installing the first extension.