Shojikinaai the host needs to open the ports given on our doc in his router with the udp protocol.
Afterward he can start the game and start a game with the solo button (it's an online multiplayer game, it's not solo)
Once he is in-game playing with a weapon etc. the other player can join him by double clicking his name in the Plutonium overlay friend list tab (F10)
If by doing this the other cannot join then the host either did the port forwarding wrong or a firewall on his PC is blocking the connection