Assuming you're on the site and not on the app, just make sure the left-most tab (pictured below) is clicked -- there you'll see your chat, along with a couple of bits of game info. The text-box where you actually type is at the very bottom.
Not sure what the default starts out as, but if you go to Settings/Live Chess, there's an option that can enable/disable the chat, along with whether or not you want only 'friends' or 'everyone' or 'nobody' to be able to chat with you, and one or two other chat-related options.
Yes, assuming your and your opponent's options are set correctly, you can chat during (and even after) your game -- just be careful not to waste too much time, of course.
And i think there's also a little button that pops up in the right-top corner of the chat box that enables you to quickly disable/block the chat during the game...
I've read on the forums about chatting during live chess. I cannot find it. I have not been able to turn it on or off while playing live chess (10 minutes per side). Nor do I know how to access it. Is it on or off by default? How do I turn it on or off? Can I read it during the game? Can I respond during the game? As you can tell I don't know the first thing about how to engage in chat during live chess. Any help that can be offered will be greatly appreciated. Thank you.