I'm having exactly the same problem with ATC. So far I haven't been able to find a solution but I'm still looking.

I found that jfw0371 had a post that solved my problem (and I feel rather stupid). I needed to turn on the radio (the ATC window will open if the radios are off, but you can't use them). I got to the radio stack via shift+2 (the radio button is the tower icon). Clearly I need to think more like a pilot and less like a computer gamer. Also, manuals and checklists. Oh well, live and learn.

If you get the Start-up tutorial from the KB at the Support page, it will tell you to turn ON the radios not too long after turning the battery ON. I have got into a habit of turning them on after turning on the panel lights. For me this is the third step (battery, panel lights, radios).

I actually found a good start-up tutorial, which I prefer to the one mentioned above, by doing a google search. Same as above when it comes to radios. It tells you to turn them ON not long after the battery.

We all live and learn.

Mark