Hi all, having just gone through some heart-ache with trying to work the radios (contacting ATC like everyone appears to be having trouble with), I can say I've found a solution.
Assuming you have the correct frequency, radios powered, throat-mike off, AM selected etc, you need to access the correct radio menu. I.e., accessing the radio menu from the normal '\' key doesn't cut it. You need to use the actual cyclic radio trigger button, which will have the radio type as a heading when selected (the other menu won't have anything).
No doubt many will have this worked out already, but I haven't heard it mentioned in the numerous radio threads, and thought I would list it here.