According to this page:
http://help.twitch.tv/customer/portal/articles/1302780-twitch-ircthe server is irc.twitch.tv, following the instructions here to obtain an oauth key and then connecting to the server (it gives you the exact line you need to connect with) worked perfectly for me. Trying to connect to tmi.twitch.tv does not work. Make sure to /nick yourusername before connecting, or use the -i switch to specify it when connecting.
You can then bring up the options, then go to the server list and add it like this:
Then select it and all you need to do is type /server twitch to connect.
If you want to connect automatically when you start you can do so within the options under Connect > Options. If you want to join a channel when you connect you can do so by opening the favorites window (there's an icon for this in the toolbar), then entering the channel you want.
Or you can do this with a remote script, open the script editor with alt+r+r and then place the following:
on *:start:{
server irc.twitch.tv 6667 oauth:youroauth -i yourusername
}
on *:connect:{
if ($network == twitch) || ($server == tmi.twitch.tv) {
join #somechannel
}
}
