The Sims 3 Generations Expansion Pack won't install?

I just bought the sims 3 generations and it won't install. i bought it at target, so it's just a regular PC disk. whenever i try to put it in & click run/install, it doesnt work & a little warning sign thing comes up & says: "This product requires an installed copy of the sims 3. please install the sims 3 before installing this product." i've never installed another sims game on this computer before, so does this mean that I need the regular sims 3 first? if anyone can help please respond! thank you