This is the method that I've always used. It's pretty simple. Download the Teleporter from Simlogical:
http://www.simlogical.com/sl/Sims2Pages/Sims2_Teleporters.htmScroll down to Teleporter Basic and Teleporter Plus and select the one you want. Then, once in the game, you can use the teleporter to
summon the sim you want to move in. It might be a bit of a pain to find the sim if he's a townie. Sometimes townies show up under random last names, rather than their own, so you may have to hunt for a while before you find him. If he has his own place, just look for his last name in the list. Anyway, once you summon him (he'll show up in front of the teleporter), click the teleporter again, and select
move in..., and select the sim's name. Now he'll be a controllable member of the house. After that, you shouldn't have any trouble marrying the two.