Ooooh Wait... Clue word... Riverview isn't in the dropdown menu....
Hmm... let me take some troubleshooting notes.
That may mean it was uninstalled somehow.
Have you checked your Downloads folder to see if Riverview is in there?
\Documents\Electronic Arts\The Sims 3\Downloads
If yes, check the launcher to see if it's in the Installed tab? Is it there?

If no, it may need to be reinstalled.
If you have Riverview in your Downloads folder, double-click the file and it should automatically start installing.