The bonus display on Zombie snatcher receives its data from the computer and is physically connected to the I/O board located in door 3 (com port 1). If the communications ports are reversed the display will be blank because the computer is sending its output data on port 1 which has no physical connection to the display board.
The easiest way to determine if this is your issue is to play the game and win a puck. When you win a puck the log portion on the bottom of the screen displays the puck information and the station that won the puck. When the com ports are reversed the station numbers will be reversed so if you won the puck on station 1the log will display that you won it on station 4.
To correct the problem turn the game off and reverse the USB to serial port connections on the back of the computer or see the PDF below showing how to reverse the com ports in windows.