Always Getting "We are unable to verify" Error!

[lilkitty478]

I just bought Sims 3: Late Night from Walmart not too long ago. Everytime I start it up I get "We are unable to verify that your disk is a valid copy of the Sims 3: Late Night. Please contact customer service." It has worked sometimes when I turn off my Mac and hold the power button, Option, Shift, and Control buttons to turn it on but that has stopped working. I don't know what else to do :/ Could it be my Mac? It has happened to Sims 3 and the Ambitions expansion pack also. So please help!