Mobile Base Crawler

You will have to go inside the "Enclave Mobile Base" and fire an airstrike to complete "Broken Steel" (part of the last quest given for the DLC) Once inside you have the option to launch an airstrike on the Andrews Air Force Base or to start an airstrike on The Citadel. There are three other locations mentioned as orbital strike options, Rivet City, Megaton, and Project Purity, but if you click on those it will just have an error message.

By calling in an airstrike on either location (Andrews AF base or The Citidel) you will still end Broken Steel.