Ask support from them perhaps? Ask if they support the integration with xenforo software. As the passwords are stored in a different table than the users, and dont just use "md5"
It's a bit scary that there is software out there storing passwords in only md5 hashes. Todays rainbow tables are massive.
You sure your not at the install screen, were you have to set-up a different database (from forums) beforehand it will use, then enter that information into that install screen? Then as for User Name and User Password, those get created on install so you can enter what you want there for yourself, use same as you use on forums for name and password for example?