downloaded houses

okay so I've been working on this for about an hour now & I'm starting to get frustrated. I'm on a MacBook Air & I've tried downloading like 4 or 5 different houses, all separately. to use in the game & I've placed them in the tray folder & I can't get them to appear in my gallery at all. I've looked it up & it seems like I'm doing everything right but for some reason they still don't work. does anyone know how to fix this?