By default DCS adds a key/key combination/button.
Make sure both of you are actually running TARS. You can check that once in the pit, tab out to TS and click on you and your friend. It will show up under his info.
If tars is not running, check that you installed it properly and that tars plugin is checked under teamspeak > plugins.
If tars is running than both of you should turn on the radios and be on the same frequency.
Prosper!