I just did mine a couple weeks ago and it took me awhile to figure how to do it but all I did was follow the instructions from retroflags website.
https://github.com/RetroFlag/retroflag-picase
scroll down and you'll get to the instructions for recalbox ...my mistake before was i started downloading and tried to install the script through the config page, didn't work.
follow the instructions step by step.... and you'll need to plug in a keyboard to your raspberry and not to the case
for RecalBox:
Make sure internet connected.
Make sure keyboard connected.
Press F4 first. And then press ALT-F2 enter termial.
User: root Password: recalboxroot
In the terminal, type the one-line command below(Case sensitive):
wget -O - "https://raw.githubusercontent.com/RetroFlag/retroflag-picase/master/recalbox_install.sh" | bash
that last line make sure you type that whole thing
