pets install issue

[rooney]

i've been playing sims2 on my macbook for a month. no problems. i bought the pets expansion pack. when i try to load it onto my book, an error message appears saying i need to re-install sims2. i've done this twice and i still get the message that sims2 can't be found. could it be something simple as a dvd/ cd rom compatibility issue (sorry about the spelling).
i'll try anything

[Sacharissa]

Did you uninstall both Sims 2 and Pets before reinstalling?