Put the "vpk.exe" file in your "common\Half-Life 2 Deathmatch\bin" Folder

E.G. something like this.
C:\Program Files\Steam\steamapps\common\Half-Life 2 Deathmatch\bin\vpk.exe

Drop your new own custom made addons folder on the vpk.exe.

E.G.
If you made a folder called "my_custom_addon" with your "materials" folder 
set up correctly inside just drag and drop the folder on the vpk.exe.

You should now have a new file called my_custom_addon.vpk, put this file here.
C:\Program Files\Steam\steamapps\common\Half-Life 2 Deathmatch\hl2mp\custom

Load you HL2DM and your custom "my_custom_addon.vpk" will load up.

P.S.
Custom fonts (.ttf) e.g. for Crosshair are not supported for .vpk 

