I presume that your Marriott Rewards earning preference must be set to ‘airline mileage’ rather than Marriott Rewards point for this promotion to apply. Perhaps switching your preference and then trying to register might work?
John – existing MR members should be able to register by signing in to the MR site via the above link.
Please do report back if you are already a ‘mileage earner’ and you are still having trouble registering. If that happens, then perhaps someone from Marriott might read this and sort any bug(s) out.