[MAME] Carnevil

Hey, I'm playing carnevil on mame, but I'm having trouble with the lightgun calibration. I press F2 to get to the service menu and the '-' and '=' to choose different options, but I can't find the button that allows me to select anything! Maybe it's because I'm running on an laptop and there's fewer buttons. I read the documentation and the ingame help, but still couldn't find it. Any ideas? Thanks.