You'll see 5 "radios" when you press the / key. Each radio has a pull-down menu, so use your mouse to select a channel other than "All" for the 1st radio on the left. Most people select the "Country" channel for that one.
To tune to channel 200, you would choose any other radio except for the V channel (that is for Voice) and type 200 into the channel box. Players from all countries can see the 200 channel if they have tuned to it.
Check out the trainers information link in my signature for further information. Welcome to Aces High and have fun!