change "firmware directory"? There's no setting by that name in RepG. Do you mean the "Advanced" Preference that sets the "Firmware update URL". And you cannot run RepG from the .dmg file. You need to drag the ENTIRE folder out of the expanded DMG image to your Desktop, /Applications directory, or similar. Do not just pull the RepG executable out of that folder.
And also make sure it is RepG 40r33 - Sailfish (or later). "r33" being an important aspect.