Multiplayer Maps Not Appearing
For a level to appear in the Half-Life 2: Deathmatch map list, it must have an
info_player_deathmatch in it.
info_player_deathmatch entity creates a player spawn point for multiplayer games. When a player spawns in a level (either because he is just entering the level, or he has just died and is respawning) his location will be picked randomly from all of the available
info_player_deathmatch spawn points.
Its a good idea to include a large number of these spawn points to prevent predictable spawning.