If your cert is an e-cert, you should be able to see it listed in your online account. Assuming that is true, I do believe, you can navagate to your reservation and since there is no e-cert currently attached, it will let you attach an e-cert that you currently have.

If you are having trouble doing that, or not sure of what you are doing, just give MR a call and they can easily do that for you as well.