To enable master keypad operation, check the Enable master keypad option. You can set the number of the master keypad to any keypad you have, but it will not be able to vote. A low numbered keypad is recommended.
The table in the middle of the dialog allows you to customise the action taken when the specified key is pressed. (In some restricted licenses, you cannot customise these). For example, the standard is to have the master keypad key 1 move one step forward in either Impact Explorer or PowerPoint. You could move this to the 0 key if you felt that suited you better. All of the options are available by selecting a line, and then clicking on the down arrow to drop down the list of possible actions.
Note that the response time of the master keypad is no better than the other keypads, so if the system is fully loaded with keypads, it may take up to 5 seconds for the master keypad to be acted upon. For this reason, you should (if possible) keep the master keypad in the first 50 if less than 250 keypads are in use.