I just had the same issue as damnedcat777.
This procedure helped:
• Disconnect Keystep.
• Download latest firmware first.
• Open both Midi Control center and Audio MIDI setup.
• Delete "Arturia Keystep" entry in Audio MIDI setup / MIDI studio, as well as any generic "USB MIDI Device" entry and/or "KeyStep Updater" entry
(Note, that a connected item cannot be deleted).
• Connect Keystep while holding REC/STOP/PLAY down. You'll see the Hold, Shift and Oct+/- buttons illuminate a rotating circle. This says BOOTMODE.
• Now follow the steps given by the Midi Control Center. Use locally downloaded firmware copy.
Worked here like a charm. Note: We coudl have spared all those messages, if Arturia's FAQ entry would be a LITTLE more detailed.