EMPShielded The launcher's Install button is supposed to be for packs you own but haven't downloaded yet, which doesn't apply here. It sounds instead like Supernatural is installed but the launcher isn't recognizing it, and reinstalling it won't help.
Instead, please do a clean uninstall and reinstall of Sims 3, which will remove registry entries and other hidden files that might be preventing Supernatural from loading. Here's how:
[Read First] Running Sims 3 in Windows | EA Forums - 8323446
You can reinstall just the base game and Supernatural first, to save time, if you prefer.