Link:
http://15thmeu.net/forums/index.php?topic=19662.0Some quick easy steps to ensure your server is visible in Steam.
SERVER CONFIG
1.Delete any previous reporting IP's or addresses in your server config or TADST (leave it blank)
2.Ensure your ports are set to: Port: 2302, Steam Port: 8766, Steam Query Port: 27016
3.Disable Windows software firewall and any other software firewalls
ROUTER CONFIG
Port Forwarding:
1.Arma Server Ports: UDP 2302-2305
2.Steam Port: UDP 8766
3.Steam Query Port: UDP 27016
Give it a minute or so, your Arma server will appear in Steam Multiplayer.