Hello Erik,
Are the customers importing into Method correctly? Are the records you imported in the Customer and Contact List and can you open the record? It sounds like there is a problem with the sync between Method and QuickBooks. I would suggest to give Method support a call at 1-888-925-6238 x2.
There are some things you can check. Are you getting errors when you do a manual sync? Are there conflicts in your list? (Go to QuickBooks -> Resolve Conflicts)
Greg