For regular Ubuntu versions you can download the Steam installer right from Steam and double click it and it will open software center. With blackbuntu based on an old version of Ubuntu it may not even run Steam.
Just curious, why would you install Steam on a distro based on Ubuntu 10.10 that was designed for penetration testing? The webpage says clearly:
Penetration Testing Distribution based on Ubuntu 10.10 which was specially designed for security training students and practitioners of information security.
Does this seem like the best choice for a gaming platform?