Basically in the import zip there are a couple of files, postlist.php, showflat.php and showthreaded.php. Each are commented with instructions as to what needs to be changed
Place those files in your OLD forum directory (replacing the current ones - you can rename or save the old ones)
Change the path to point to the new forum location.
You may also need to set $search_friendly to 0 in each of the scripts to disable the search engine friendly style URLs if those are causing a problem.
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