At the top of the forums page, there is a drop down menu labelled "Chat". You can either just click on that, or hover over it to open the drop down menu, and choose one of the chat clients to use to enter chat. I would recommend NOT using the "Mobile Chat" option, unless you are on your phone.

I usually create the room for the chat about five minutes before the time the chat is scheduled for. Feel free to join whenever you wish!