Canadian carriers must call DAT at 800-551-8836 in order to get assigned a DAT ID number prior to registering. Since Canada doesn't have a DOT or MC number assigned to carriers, a unique ID number will be assigned to you upon your call. Once the ID is assigned, you will be able to register with it.
Do you have active authority?
DAT requires that you have active DOT or MC authority (unless you are Canadian) in order to register for DAT OnBoard and haul loads for brokers or shippers. Once you show as active in the FMCSA, you will be able to register for DAT OnBoard.