Dunno if this will help, it's a quote from Interfection in
this thread. Try the same setup with your downloaded world and see what happens.
"To anyone that's having trouble with playing games saved in Riverview,
Put the Riverview.world file in C:\Program Files\Electronic Arts\The Sims 3\GameData\Shared\NonPackaged\Worlds like Witch instructed. Next, you must delete the Riverview.world file from your InstalledWorlds folder in your Documents to get it to work correctly."