Included in The Gamer's Kit for Raspberry Pi is an SNES-styled game controller. It has 12 buttons in total. That's enough buttons to play many retro games on various emulators for GBC, GBA, NES, SNES etc.
As Retropie has already been pre-installed, you will see the Retropie splash screen on boot up.
After the splash screen, you will see 'WELCOME'. This is the menu that will take you through configuring the controls.
Hold down any button on your controller or keyboard and the name of it will appear. "... GAMEPAD DETECTED" will appear at the top.
Follow the onscreen instructions to configure the game controller.
If there are not enough buttons on your controller (the SNES controller in the kit has 12 buttons), just hold down ANY button to skip a control. Finally, when you get to the 'OK', press the button which you have configured as "A"