Half-Life Deathmatch: Source runs on the Source engine, so you admin it over RCON, not SourceMod chat commands. The easiest way is the web console on your server card: type a command and hit send. You can also do it from the game.
From the web console
Open your server on the console page and use the buttons (change map, mode, frag/time limits, gravity) or type any command below directly.
From the game (client console)
Enable the console (Options → Keyboard → Advanced → Enable developer console), then:
rcon_password yourpassword: authenticate once (the RCON password is on your server card).rcon status: list players and their IDs.rcon changelevel crossfire: change the map.rcon kick "name": kick a player.
Useful settings
mp_teamplay 1: switch to Team Deathmatch (0 = free-for-all). Applies on the next map load.mp_fraglimit 30: end the round at 30 frags (0 = no limit).mp_timelimit 20: end the round after 20 minutes (0 = no limit).sv_gravity 300: low-gravity chaos (800 = normal).mp_friendlyfire 1: team damage on (only matters with teamplay).
Maps
The stock maps: crossfire (the classic, with the nuke button), boot_camp, bounce, datacore, frenzy, lambda_bunker, rapidcore, snarkpit, stalkyard, subtransit, undertow.
How friends join
Share your address; in their console they run: connect 1.2.3.4:27015. If you set a join password, they add it: connect 1.2.3.4:27015;password yourpw.
What does not work
There are no working bots for this build, so the server needs real players. It shines with friends. Custom maps are not one-click yet (they need a FastDL host), so stick to the stock maps for now.