Just out of curiosity, have you tried pressing the start button (in MAME it is usually mapped to the "1" key)? Also, you can press Tab and see the key mappings, and try them one by one. You may want to check the Dip switches, too and see if there is one that enables this.
The NES version is a little different. It could be that this feature was available on the NES version instead of the arcade.
Additionally, there is a website that has more about the game:
www.rbibaseball.com