Midnight Hollow download and installed but can't access it?

scroll down further in the list to see if your world is there as that sometimes happens the buy one is at the top the installed one is down further also when in game turn off the ingame shop then exit out  relaunch and check the list again

see here http://sims3.crinrict.com/en/2013/05/ingame-store.html

and if that doesn't work see here scroll to  I installed my Store Content but it's not showing up in my game

http://answers.ea.com/t5/The-Sims-3/FAQ-The-Sims-3-Store/m-p/333394#M155147