I believe you're looking for the program to create maps, am I right? To do so, open Steam, go to the Tools tab, install or open the Left 4 Dead Authoring Tools (If you cannot see it, you do not own a copy of L4D on your current account or it's a bug and relogging to Steam fixes that). When you have done that, a new window will pop up, double click on the Hammer World Editor, and that is the program to create maps for L4D. You can read the guide by double clicking on the Getting Started Guide.
I am having a little trouble with a few of my L4D reskins. Please, PLEASE take a look in this thread.
Once I get these 2 problems fixed, I can release all of my reskins!!!