Late Night won't install??

I downloaded the starter pack a few days ago from Origin (no disk). The base game and High End Loft Stuff works great, but when I try to install Late Night, it says the product requires the installation of the Sims 3 base game, which is already installed and works fine! It won't recognize the game, and I've gone as far as uninstalling everything and installing it again, which took hours. Help?