Air traffic control
There is the limited possibility of ATC with v0.9.10 and v1.0.0. To find out how ATC should be used take a look at the ATC Tutorial.
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" to 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" at the multiplayer server with using the ATC-Aicraft. You can communicate with Menu -> View -> Chat or using predefined messages 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.