No, there is not another way to instal the extensions other than through the adobe extension manager.
One thing i would try is to rename both the DW and extension manager configuration folder.
see the Common installation issues PDF for details on locating the Dreamweaver Users configuration folder:
instead of deleting the cache file inside the configuration folder, rename the configuration folder.
In the Adobe folder, there is also a folder for the extension manager, this will lead to the extension managers configuration folder, rename that folder as well, then try loading the extension manager again.