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 start an airstrike on the "Andrews Air Force Base" or to start an airstrike on The Citidel.

The are 3 other options to start an airstrike on are Rivet City, Megaton, and Project Purity, but if you click on those it will just have an error message.

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