The Broken Steel add-on first became available two days ago via Games for Windows Live, only to be taken down due to a DRM issue said to originate from Microsoft's Live service. Kotaku has now confirmed that the downloadable content has returned to the service, with our copy in the middle of downloading as we speak. Hopefully this version will fix the previous version's not being able to be played at all bug. Fingers crossed, everyone!
Thanks to Dylan for alerting us to the re-availability.