I'm not sure there is one other than at DVC website. But, I've been wrong before!! The only time I have ever registered with DME online was within the DVC site.
Also the disneyworld.com website has, I think, the online link. Or the OP could check one of the several Magical Express threads 'stuck' at the top of the Transportation board for the toll-free number. I know it starts with 866.