Did you already give the URL to the new forums before the import was done? What happens is the very first user to register is the admin user, so someone registered before any of your users were imported and became an admin.
Personally, I'd recommend starting over, either in a new unknown directory, or if that's not possible, put it behind some .htaaccess file that requires authentication before you can get to anything in the directory to keep unwanted people away.
