After starting up, why does VoxATC X instruct me to contact a center or approach ATC unit when I am on the ground at the departure airport

This can occur when you load another flight (situation) with VoxATC enabled or if you move to another airport which is not the departure airport in the flight plan saved with the current flight (situation).

Ensure that when you enable VoxATC, you are at the departure airport for the currently loaded flight (situation) and that VoxATC is not enabled when you load a flight (situation).

My add-on aircraft are not recognized

After installing new add-on aircraft, you must run the VoxATC Indexer so that VoxATC can run work with the new aircraft.

The VoxATC panel is not available in an aircraft

Run VoxATC Panel setup and ensure that the aircraft you are using is selected to have the panel installed.

Is a disk version available

Yes, contact RC Simulations for more details.