I'm glad you got it figured out. The registration code is pretty much the same as in vb3.8.X, so I guess you're coming from a version that's older than that.
Anyway, there's an article on the datamanagers here:
www.vbulletin.org/forum/showthread.php?t=174090 which I just found so I haven't read it myself. Also if you're not familiar with the concept of classes and objects in php you might want to look at that:
http://us2.php.net/manual/en/language.oop5.php
BTW, set_userfields is a function that's defined in includes/class_dm_user.php, but you probably need to be familiar with php classes and objects to understand it.