When you are finished building your apartment lot and are ready to change from Residential to Apartment, here is what you do:

1. Open the cheat box window (CTRL + Shift then C)
2. Type changeLotZoning apartmentbase
3. Save and Exit the lot into the neighborhood. You will see the apartment icon over your lot now!
4. Re-enter the lot and you will see the apartment mailbox in place of the standard house mailbox.

A few people have contacted me about Apartment Mailboxes not showing up in their game after doing the steps listed above. If this is happening to you, don't panic! You might be one of those who has downloaded recolors of maxis mailboxes in your game! I know I ended up with some and have no idea whose lot they came with. But, here is how you can get rid of them:

1. Go into your Sims2Pack Clean Installer, if you don't already have it, you can download it at http://sims2pack.modthesims2.com/
2. Choose directory as downloads and then it may take a long time for all of your downloads to appear, depending on the size of your download file. Good time to go make a cup of coffee or dinner, in some peoples case!
3. Alphabetize your downloads by clicking the name bar.. that won't take as long at all.
4. Scroll down and delete ALL mailboxes you find.

Don't worry, when you reload your game and go into houses, the mailboxes will be replaced with maxis mailboxes!