I added a plug to my garage (both lights and plugs are on the same breaker and the lights are three-way). Once connect everything worked except - I can't turn off the lights using the switch. What have I done wrong?!

Well, you have to tell us exactly what you did when you added the receptacle.

Where did you tie the receptacle in... at one of the switch locations? Do either of the switches work? Can you provide more details please?