did you followed all the instructions available here:
http://wiki.flightgear.org/FGCom_3.0 ?
Especially the testing part.
Please be more precise on what you achieved, what does work and what doesn't work (echo test ? testing with multiple pilot ?)
Also you have to tell more about your configuration:
* Windows XP ? Win 7 ? Win 8 ? Linux ? Mac OS ?
* are you using FGCom builtin ? standalone ? which version ?
* do you see your callsign in the user list at
http://fgcom.flightgear.org ?
* did you tried with multiple pilots ? always the same problem ?
* how is your audio installation ? USB headset ? speaker ? jack headset ? FG sound works fine ?
The most important things to test being the "echo-test" (read the documentation in order to know how to test)
regards,
Clément