Sims 3 step 3

Hello everyone , my name is gabi and i am new in the Forum.

i have a mac book pro. and i been trying to install the Sims 3 for a while now. but somehow i **** up the step 3 and i didn't install the expansion pack and i crack the game , so i was wondering if anyone can help me to go back ? or i need to download everything again , because when i try to install an expansion pack tells me something that the game is not compatible for update and it just close .
hope you guys can help me with it , as i am coming back to the Falklands Islands , and the internet is so expensive i wanted to have a cool game to play when my hubby is playing ps4 or w/e ( never too old for sims xD ). thanks very much for your help !

cheers from Falklands Islands