You probably installed it at some point and have left over files.Either via the launcher or in your user folders.
Go to --- Electronic Arts --- The Sims 3 ---Installed worlds
and see if you can find a file with the world name.If you do delete it,
then clear your game's cache files.
Start the launcher and scroll through your installed content and see if you can find
the world.If you do, click it, uninstall it, then reinstall it.If it doesn't show up, try
just redownloading it and reinstalling it.