A RKDR 2 Retropie sd card image can be downloaded here
To use the pre configured image you will need an 8gb (or larger) Micro SDHC card and Etcher
install the etcher software
select the micro SDHC drive
select the downloaded image file
select start to write the image to the sd card.
when complete, remove the card and insert it into the RKDR device
power on the device