Crashing at final door

[heartsauce]

So I purchased the Penumbra games on Steam and am using it on a unibody Macbook Pro 13-inch. The game has been running great this entire time.

I just burned up Red, and after I pry open the door with the crowbar, I see the loading screen narrative for a second or two and it crashes.

Apparently this is a common crash, and I downloaded the Mac patch that is available on the Frictional website.

After applying the patch, I am prompted for a serial number. Because I bought the game on Steam, none was supplied. What do I do?