Roll Call Mobile CheckIn runs inside your mobile device's browser. It's a web app with nothing extra to download; you have less to worry about!
Basically you will open your browser and connect to the IP address of the computer that is hosting your Roll Call database. As long as you have Mobile CheckIn running on that database (and assuming you're on the same network), you should be able to log in to your mobile checkin user account and perform check-in from your mobile device.
Please see this FAQ for more info.