/join servername.channelname
So for the alla chat channel, it would be
/join blackburrow.asylum
be warned though, it isn't that stable, so so it may not always work right. Sometimes you may be able to see the chat, but it wont go through if you try typing something. Or sometimes it will be like only people from other servers can chat, even though there are people from the server it is on in the channel (through a whochannel).