If you are using one of the latest versions of X-Plane, the simplest way to interact with ATC is using the Nearby Air Traffic Controllers window. You can open it by going to the Special Menu and looking about halfway down the list. You can auto-tune your COM1 radio to the frequency of the controller you’d like to contact by clicking on its row in the list. In addition, if you’re ever en route and not sure which controller you should be talking to, X‑Plane will do its best to suggest the right one—you’ll see a little asterisk (*) next to the recommended controller.