Hi, we are trying to play in a pm, so we did like the 4D1 tutorial.But when he tries to connect to my ip he gets unable to connect to host!How can we solve this problem?

Have you port forwarded 28960 on tcp and udp? And any fire walls you have maybe blocking the 28960.

Does he even provide the port while connecting? Its supposed to be "/connect ip:port"

You can use Hamachi if you don't know port forwarding

 

also check your firewall

You be better off setup a password protected dedicated server.