you may need to recreate the users configuration folder. see step 12 of the adobe troubleshooting guide:
http://helpx.adobe.com/dreamweaver/kb/troubleshoot-javascript-errors-dreamweaver-cs4.html#main_user_config
after renaming the configuration folder, download the Design Extender ZXP file again, if you are prompted with an option to open the ZXP file or save it, make sure to select the option to save it.
the open the Extension Manager, click the install button and browse to the Design Extender ZXP file that you downloaded.