Re: Rental units not working
Are you trying to open the apartments in build mode or live mode? In live mode, you can't enter a lot that's vacant as you have to knock on the door. In build mode, you should be able to open the lot from the map by clicking on the build icon in the lower right corner of the screen. You should go into the lot in build mode first and make sure the venue is set to residential rental and that all the rooms inside have been assigned. Otherwise, they will all be assigned to shared and the lot won't work properly.
Once you've got all the apartments set up, then you can move the sims in using Manage Households or go to live mode with the sim you want to be a landlord and have them buy the lot. Then the landlord has the option to pick tenants for each apartment and they will move in on their own. If you want the game's default landlord to run the place and play as a tenant, then just move sims into the apartments via Manage Households.
Hope this helps.