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.
If the chat icon is disabled in the member's profile, this means that the person is not available to chat