Hi Nat
I'm sorry to say that this seems to be an increasingly common problem, particularly with worlds. I'm not sure why. You may already have tried this but if not then see this excerpt from BluebellFlora's blog:
Downloaded content not showing up:
If your Store downloads aren’t showing up in game try deleting the missingdeps.idx file (thanks to Crinrict), the game will regenerate a new one:
Finder > Documents > Electronic Arts > The Sim 3 > DCCache > missingdeps.idx
This isn’t a sure fire fix as Origin seems to be causing a lot of issues with the Launcher but it’s worth a try. For more info read Crinrict’s blog post here.
Caspin