I’m playing with the downloaded file. I now tried on Windows. Same problem. What is the Start key? I press space on all screens before and that works. But on the charactrer selection screen it does nothing.
To navigate in the menu you can use the "enter" key to validate, and "escape" to cancel.
For the character choice menu it is different because you have to use the controls assigned to the players, the key to press is normally indicated on the respective panel of each player.
You can also play with a gamepad, and change the keyboard controls in the "controls" menu
I can confirm that the Enter key does nothing. This is on both Linux and Windows and on the same machine (a new latop I bought last summer). I will try on my macBook tomorrow. There is no other issue with the Enter key anywhere on Linux and/or Windows in any app so this is as WTF as it gets, I guess. :)