Air traffic control
There is the limited possibility of ATC with 0.9.10 and 1.0.0.
Frequencies
Menu ->ATC/ AI
set the right frequencies and around your choosen airport you can read (or here with festival) the ATC-Messages. For that you have to check under options "enable ATC" If you check "enable AI Traffic" too, you will have some AI traffic (c172) around the airports.
ATC Chatter
Menu ->Sound Configuration Check "ATC Chatter"
You will here some random records of real-ATC. For the right feeling! ;-)
Frequency
Set the right frequency to COM1 with '-key you will have the possibility in some areas to send messages
ATIS (Automatic Terminal Information Service)
Set the right frequency of the ATIS to COM 1 and you will hear the ATIS for the choosen Airport (unfortunately a little bit low)
Multiplayer
Sometimes someone plays the "Tower" with using the ATC-Aicraft- you can communicate with Menu ->view -> chat or using predefined mesaages with Menu-> View -> chat menu. You can use FGCOM too!
There is not yet an artificial-interactive ATC implemented, so the ATC-system in FlightGear is still limited.