Just installed and tested this plugin.
Had no problems with it.
Wanted a different look for posts in the category "news" so set my custom permalinks as: /%category%/%postname%
Then in Settings >> Multiple themes plugin
... under "For An Individual Page, Post or other non-Admin page"
... under "URL of Page, Post, Prefix or other" pasted:
Then in the same field entered:
... and checked the box "Anything that begins with this URL will use this Theme"
Now my "News" category page and all posts in the "News" category have their own distinct theme.