Go through and verify that in EVERY group you have defined, none have the NEVER permission set. Also verify that the nodes (forums) have no specific user permissions set. If so, remove them and then start reconfiguring them.
That's no problem.. you will find there are several users (and moderators/staff) that are not "official XF employees that are more than willing to help. @King Kovifor, @Brogan, and several others. With it being an import from VB4, @Jake Bunce will be another good resource. It could be something to do with the importation of the DB?
Lesson of the day for those who may stumble upon this:
Make sure you complete your import (very last step) before attempting to 'open' and let the necessary rebuilds occur. Also, verify that all user groups have the "View" and "View Node" permission (or else they won't be permitted to view your forums). One last step, make sure you rebuild your search indexes.