Since i don't use the Chat feature enough.. i'll give you my limited knowledge of it..and maybe someone else can add more.
On the bottom right of your screen you will see a "bubble" looking Icon.
Click on that and make sure the box is checked that You are available to Chat.. and then just follow
what the Tip says.
Tip: To start a new chat click on the "chat" icon from another members' profile.
When you see the person's profile, he has to be on-line and actually active on his computer (at least in the past minute); you will then see the chat icon available. Click on this, and the chat window will appear on the bottom right corner of your screen. Then, just type your message.