Click edit then put in the ip for whatever network it's on. The if you favor the network on your network list it'll connect to the network when you start xchat and join the channel.

First of all, unless you're using it on purpose, update your Xchat. You're using an old version of it and the new version is a bit more organized for defining networks, and makes dealing with nickservs easier.

Secondly, you're trying to connect directly to the channel. That won't work. You need to connect to the GeeksIRC server, then to the channel. Remove the address you have in the server list now and replace it with "irc.geeksirc.net/6667";. Then, under Autojoin Channels put "#tf2.mix.nahl". Uncheck Use global user information (it seems you've already done that) and under Login Method select "Nickserv (/MSG Nickserv+password)". Then put your nickserv password below it. Note that if you haven't registered a nickserv yet, you need to do so at the GeeksIRC site using the registered name you used on Gamesurge if you want to keep your stats.