This is the simplest and easiest way to set up a dedicated server for Half-Life Deathmatch:Source.
Note: You Must Own Half-Life Source/Half Life Deathmatch:Source to do this.
1. Download Source Dedicated Server - If you do not already have this on steam, go and download it. It is free and is in the tools section of your main steam window.
2. Go and download GCFScape - Download just the installer here.
3. Extract the GCFs - Extract all the files from the GCF files half-life source.gcf and half life deathmatch source.gcf, which are located here: C:/Program Files/Steam/steamapps/
Extract these files into the following folder: C:/Program Files/Steam/steamapps/YOURUSERNAME/source dedicated server/
Note: If you have a 64 bit operating system like I do, then "Program Files" will be "Program Files (x86)" because Steam is a 32 bit application.
4. Launch Your Server: Go to your steam main window and go to the tools section. Click on Source Dedicated Server, which you should of installed earlier, and wait for it to load. Select under the "game" menu "HALF-LIFE: Deathmatch". Then choose your map and other server options and, finally, launch your server!
It's that simple!