Before you click the 'Start Flight' button, go to your Advanced Features in the Mac Launcher by clicking on the little triangle.
Select the 'Others' page.
Click in the Command-line options box.
Type in --com1=122.75
That is the most commonly used radio channel. There needs to be another FG user's aircraft within range (50 nm) for you to hear/talk etc.
Press your space bar to talk.
In the 'Network' page, both 'Enable Multiplay', and 'Enable FGCOM in Multi Player mode' should be ticked/checked.
If you want to check if FGCom is working when you cannot hear anyone else, replace "--com1=122.75" with "910". That is the bounce box. Press your space bar and say something - you should hear your own voice in your speakers/headphones after a very short lag (less than a second).
I hope that helps.