how to play coop-missions?

[David198502]

hey guys...
i just wanted to play a self made coop mission with a squad mate, but unfortunately it doesnt work.
if i create a server, and click start battle, then the map will show up, and also the flags.i can select my spawn point and start playing the mission...but my mate cant join.

its the same when he creates the server and i want to join.it doesnt show up in the clients list.
i have disabled windows firewall to see if that makes a difference, but unfortunately it still doesnt work...