Hi Tinkerbell2726,
To start with, you have to have chat enabled: tools >options >chat, then tick 'enable chat'.
When you're downloading: right click on file, then click on 'chat with host'.
When someone's uploading from you: click on 'chat' tab (be patient, sometimes it takes a while for the chat box to appear).
You have to bear in mind that for chat to work, the other person also has to have enabled chat. Also, a lot of the time people aren't sitting at their computer & so might miss your attempt to chat. And finally, most of the time it's just a dog of a thing & doesn't work no matter what...