If when you click Play, BMS doesn't launch...

Black Mesa Archive In-game Issues (Mod)
#1

I had this trouble a short while ago. To fix it, I did the following.

  1. Opt out of the Steam Beta.
  2. Verify in Tools that you have Source SDK Base 2007 installed.

Not sure if step 1 is entirely necessary, but it’s what I did and suddenly it asked me to install the SDK Base when I clicked Play, so something in the Steam Beta may have been interfering.

Enjoy BMS!

Founded in 2004, Leakfree.org became one of the first online communities dedicated to Valve’s Source engine development. It is more famously known for the formation of Black Mesa: Source under the 'Leakfree Modification Team' handle in September 2004.