To talk in a channel click on the channel tab and type your message directly into the chatroom.
If you remain in global you will see all the chatrooms you are in and can chat in either one by first typing /h message for help channel, /all message for all channel, /t message for trade channel or /b message for the battle channel. This works for all official channels. If you wish to chat locally with players on the same map just type directly into the chat while in Global.
To chat privately with another player right click on their nickname, either on the map if they are nearby, or in the game online list, and choose 'Message'.
If this isn't working for you please explain the situation more clearly, and include what channel you are in, what command you are using if any, and where you are trying to talk.