Hi, I had the exact same problem with ANY expansion pack. I'm a Mac user, so here we go, here's what helped me.
Before installing your game, open "Disc Utility"
Most likely, "RecoveryHD" is greyed out.
Click "RecoveryHD" and guide the mouse upwards and click "Mount"
"RecoveryHD" should become bold.
This is what worked for me. I hope for the best with you.