Discourse calls chat Channels. Which you should see below Categories on the left side of the panel.
If you can’t see Channels then it means you are a level 0 user. Your introduction email / message should have instructions how to get to a level 1 users but if you want to fast track it just post in this topic and I will move you to level 1 
@bombledmonk has a good tip as well for chat. There is a button to expand the chat to be full width.
