[MAC] The Sims3 Pets Expansion Pack

I have tried installing the Expansion Pack in accordance to instructions but keeps getting the message as below:

"To install The SimsTM3 Pets you must already have The SimsTM3 installed"

"Select a volume The Sims3.app is already installed"

The Sims3 TM game has already been installed on my computer and I am unable to find The Sims3.app file. Please advice how I can address this issue.

Thank you.

KL