Check out this thread. Get the MySQL Tool and use it to restore your forums to a fresh install of Threads. That's what I did and it worked brilliantly. I understand that your backup was done using the CP in Threads, right?

When backing up double-check that ALL tables have been backed up: https://www.ubbcentral.com/boards/showflat.php/Cat/0/Number/40232/an/0/page/2#40232