Re: Installing Mods problem
If your goal is to use a WYSIWYG editor, maybe you should think about installing Tikiwiki 1.10 beta. It's pretty solid and already has the WYSIWYG editor, no doubt integrated more completely than what you could do with the javascript plugin.
Also with 1.10 beta I can get mods with no problem. There's no message about the invalid file. Maybe the mods admin in 1.9.11 hasn't been updated since the mods repository was moved from CVS to SVN. The file at the url you posted doesn't seem to be the same one as what I see in the SVN mods repository.
-- Gary