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, in most cases it just needs to be altered to set the URL to the new forums. At that point, you just need to upload those redirect scripts overwriting the old files. Then, whenever someone clicks on a link that would have taken them to one of the old scripts it will redirect them to the new one
