Hi Claire,
I'm actually getting married in the Chapel in just over a month, March 13th, 2013!!! I have been dealing with Grace. All we've had to do is speak with our parish priest who filled out a bunch of forms and sent them to our local church diocese. I requested to the diocese to send the Nihil Obstat to me instead of to Cancun. I have emailed Grace a copy and she reviewed it to make sure it was correct. I will be bringing the original forms down with me. The Nihil Obstat is simply our diocese giving permission for the diocese of Cancun to marry us so that our marriage will be recognized as a Catholic marriage the by our diocese at home. As far as we know the only other thing we need is our passports and the blood test.