Many Source games purchasable through Steam come with the Shared Resources,
including the SDK, that you will need in order to run Black Mesa. To see if you
can run Black Mesa, open Steam and look in the tools tab. If you see an entry
titled Source SDK Base 2007, you can run Black Mesa.
Installation
If you meet the Software requirements, you technically do not need anything in-
stalled. If you do not have the Source SDK Base installed however, upon running
Black Mesa you will be prompted by Steam and asked to download it.
See the page at the next link:
http://wiki.blackmesasource.com/System_Requirements