The Battle of Steeltown is progression content that is meant to be played before the end-game sequences are reached. Players with the Steeltown DLC installed will see a splash screen upon logging in the first time with a message that confirms The Battle of Steeltown has been purchased.
With the DLC installed, players who have reached level 9 will receive a radio dispatch invitation to Steeltown as soon as they access the world map in the Kodiak.
Please be aware that later in the game (after a final confrontation with Liberty) both Ranger HQ and Downtown Colorado Springs become end-game paths, at which point the world map & Kodiak navigation are no longer accessible. Save files from Ranger HQ or Downtown Colorado Springs after the Liberty confrontation do not have the option to get back to the world map, and thus can’t be used to go to back Steeltown.