If you are already on the board Ntalker, simply click on the board of the person you want to chat, if the button is green available

what if the button is not highlighted

In the lower right corner， u can find it

When a language partner or member is online (green highlighted) ,you can click 'chat now ' on their profile and a chat box should appear to you . They might answer you or not; it depends whether they are busy or could/like to talk to you or not.