I was chatting with a host, i got their IP number, and would like to send message to them, or start chatting with them, how do I do this.

If you got the IP address plus the port number they're using ( port number followed by : followed by the port number eg ***.**.**.***:6346) then you can try direct connecting to them. Click on the Direct Connect tab & enter their IP & port number. For direct connect to work, both people should not see the firewall icon & should have chat enabled.
How to Direct Connect, Browse & Chat

If you can direct connect to the host & browse their files, right click on a file > chat with host.

Were you downloading from the other host when you were chatting? If you were & can remember the name of the file you downloaded, search for it again. It'll have a green tick next to it. Providing that only the one host had that file, you might be able to find them that way. If you think you've found the correct file, right click on the file > chat with host.