Firstly, build the apartments, making sure you have finished all the building work, inside and out, before you designate your building as an apartment. Once you do, you will not be able to access most of the build menu.
Once you have finished the building and outside, such as fencing, walls etc., you can then enter the apartment cheat:
CTRL-SHIFT-C to bring up the cheat box
Enter changeLotZoning apartmentbase
Exit back to the neighbourhood
Your building should now show up as an apartment building
Enter it again and furnish it how you like. Save and exit to neighbourhood again.
Choose a Sim you would like to live in one of the apartments. OK, your furnishings have gone, but don’t worry.
Before you rent the apartment:
CTRL-SHIFT-C to bring up the cheat box.
Enter the following cheat: boolProp testingCheatsEnabled true
Walk your sim to the front door of the apartment that you want to rent. They need to be standing right in front of the door.
Clear any actions from your sim’s queue – including other sims trying to talk to them.
SHIFT-click on the door of the apartment you wish to rent and choose “Rent Furnished.” Hey presto, the furnishings magically appear!
If the action falls out of your queue, your sim probably needs to be closer to the door.
Once the apartment is rented, bring up the cheat box again.
Enter: boolProp testingCheatsEnabled false to turn the cheat off again.
Please post all support requests in the forums if at all possible