Click on the play button to play the stream from YouTube. The chat room is below the video feed. To chat, choose a nickname and type the warped words you see in the text box. Wait for a moment for it to log you in, then type in the white space below and press enter to send a message to the room.
If you are IRC savvy, you can join the chat with your favorite IRC client. Just point it toward #darngoodyarn on irc.freenode.net.
Helpful IRC Tips:
If your chat window isn't scrolling, scroll to the bottom of the chat manually and then click on the "#darngoodyarn" button at the top of the chat window. If successful, it should mark the bottom of the chat with a horizontal rule and start scrolling automatically again.
Put "/me " in front of your message to role play. (e.g. "/me hugs Jen" yields "* Brandon hugs Jen" in the chat rather than "<Brandon> hugs Jen".)
Type "/nick NEWNAME" to change your nickname. Warning: there are a lot of people on the IRC server so expect the generic ones to be taken.
If you lose connection to the chat room, the best way to get back in is to just reload the page and log in again. You may need a new nickname until the server realizes your old one is not responding.
If the chat window is too small for you, you can use use a full-screen version at http://webchat.freenode.net. Just put in #darngoodyarn as the channel.