You have to copy the save and the code.bin file into the savegame of steeldiver. You might want to copy a payload.bin (otherapp payload for your fw) into the save too as otherwise you'd have to copy the otherapp payload into the root of your sd and keep holding b while starting the game.I thought that it needed the same procedure as fieldrunners to install the save.
thanks!
edit:it has a code.bin too in the zip file but I don't know what to do with it
Please try to download the steelhax.zip file and copying the save and code.bin for your version into the savegame. Basically the same as what I wrote for PrometheusGI'm having issue with this. It keeps saying it's corrupted and I followed the directions I'm on a NEW3DS on the USA Region.