Sims 3 not working

I am running OSX Capitan 10.11.3

On my computer I installed Sims. Then immediately I bought a Sims 3 Pets expansion pack ~ not the disc but the code. I have installed it. However when I go press 'play' the Sims 3 pets icon comes up, and then quits and nothing happens.

I just want to play the game!