How do I open my 2010 Cadillac CTS with keys locked inside and a dead battery?

My Cadillac ATS (with the battery in the trunk) has a positive battery terminal post under the hood on the passenger side. Use jumper cables to connect this to another battery. This will energize all the electrical circuits in the car, so you will be able to open the trunk in the normal way. You can jump start the car this way also.

Call a lock smith, 5 mins tops

If you didn't have a spare key in your wallet then call AAA or a Towing company, much cheaper than a locksmith. If your in town and near a shopping area the police can be helpful and open the door for you using a slim jim thought the the rubber around the window.