You can start Kodi Media Center either by pressing the X button or by pressing START and selecting the first menu entry.
To quit Kodi, Navigate to the lower left power icon and select "Power Off System" in the program, and you will come back to recalboxOS.
Currently(3.3.0b17) using the "Exit" button may lock up your PI
The controllers are now supported in kodi, but if you prefer, you can use HDMI CEC or a smartphone remote application. More info on the mini how to Kodi on recalbox.