I had downloaded an apartment building called King's Court and when I placed it in my neighborhood, it showed up as residential and it didn't have a mailbox. So I went in, changed the doors to the Unique Separator doors, changed the lot zoning, saved, and went back to the neighborhood. Its showing up as apartments but there is no mailbox still. Any suggestions? Oh I forgot to mention that there's also no trash can.

You probably need a mod that liberates the mailbox and trashcan to appear in the game menus, as I don't think there is any way to get them back once deleted. You could try these:
http://daydream.etowns.net/S2hacked.htm
Although the mailbox if I remember correctly will break the recolours that came with M&G stuff, unless they updated it.
ETA: I uploaded a modified mailbox for if you have M&G Stuff:
http://www.4shared.com/file/tdJH3s-...ox_Buyable.html

If the zoning is residential then you should see the Apartment mailbox appear when you rezone to apartmentbase- however I've had problems with apartments when they are rezoned back to residential and back again. So it would be best to place a fresh copy, place a mailbox and then zone to apartmentbase.

Does anyone have the link to the modified mailbox? I really need it!

Yes, those right above your post.

Try this one, @lalakasper
http://simfileshare.net/download/29578/

@simsample thank you!!

No problem mate!