First of all, backup the folder
Now I must correct the above solution. It needs an additional workaround because the new extension will be installed but all other extensions are no longer visible. I found out that after deleting the menus.xml and installing the new extension, in the file
[username]\Application Data\Adobe\Dreamweaver CS4\[language]\Configuration\Menus\Cache\MenusDWMenu_Insert_WebAssist.xml
all other extensions are missed which were installed before. So I just copied the lines from the old MenusDWMenu_Insert_WebAssist.xml into the new one, and now it seems work.
There are some more files in this folder in which the WebAssist lines are missed. I did the check with "winmerge", a freeware tool.
There is an error message in Theme-It that some backup path is wrong:
Wrong path in [username]\Application Data\Adobe\Dreamweaver CS4\[language]\Configuration\WA_TIR. But that's another problem, I think.