I've downloaded a few custom Buildings and Apartments of the Sims Resource. I've made sure the ones I download have only used the expansions that I currently have, yet when I go to place the apartments or buildings in my game, the building doesnt actually appear on the lot, its just an empty grey lot with nothing on it?? Whats up with that?!? This is happening with ALL the custom buildings i've downloaded

TSR has its own Help Section The Sims 3 Help Forum and a similar problem (if not the same) was posted there. if i understood the moderator right you can report houses + lots that dont work so they can be checked for general buggyness or post your computer specs to cross out your computer may not be able to display them.
also patching was mentioned...are you patched?

erm just out of curiosity: ... how long did you wait to give them time to show up. maybe they are really complex and need a bit?

I'll take a look on TSR. I waited a while for them to load but i was able to press rotate and everything but nothing was there :/ I'm only patched up to Katy perrys Sweet treats pack , but all the ones i downloaded are build on expansions prior to that which i have ) (I.e nightlife, base, generations etc). i should also add I have no problems with houses , its just buildings and apartments that are working correctly .

lets see what the TSR Help has to offer to support its communities mods. good possible they also tell you to patch - so i dont know what reservations you have on patching but it fixes a lot of problems (of all sort of kinds)