I had a similar problem and here is how I fixed it.
I would go to the Mambo's administration panel. Click "menu" and then "mainmenu" links. I think Forum is the first item on your mainmenu list. Make "Home" as your first item on the main menu list and move "forum" to second or another level.
If "forum" is not the first item on your mainmenu list, then your problem is different then the one I had, and therefore disregard this post.
I followed all of the instructions one by one. Got the same "SMF not installed.." error at the config page. I checked and rechecked everything, even did fresh install twice.
I installed Mambo by using Fantastico, and installed SMF manually (both latets versions.)
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