I had this problem too – Downloaded the 5.3.18 fw from this website, extracted the files (to ProgramData\RP\Updates by default), hit the “Update” button in the “Enroll/Update Control and Devices” screen and popup message appears saying “No M1G main control update files found”. Great!
It seems the trick to doing this update is to use the “Check for updates…” item from the Help menu. Select the 5.3.18 fw, download it and go back to the “Enroll/Update..” screen and hit Update.
Shame that the “Downloads & Updates” page of this site doesn’t mention this anywhere.