I have this lock and it works fine for me. It's a stupid question, but have you tried turning the cable around the lock mechanism so that it doesn't interfere with the power cable? As I said, Apple designed the ports, including the key lock, to provide enough clearance between ports even with the widest plugs installed. Maybe you have a weird cable or something. Contact Kensington. Maybe they can help you out. If you can post a photo, I'd be very interested to see it.
Steve