We only paid in pesos, we were told not to bring US when I got married, so they haven't accepted it for a long time. There is an exchange place at the resort, or you can get some in advance through the bank like we did, but it takes the bank a little time to order it, so go a few weeks before you leave at the the latest.
Don't tip in US, the maids and staff can't exchange it, so its no good to them. Its best to tip in Pesos because they don't have to try to get it exchanged. The staff also can't exchange CDN change, only bills, so keep that in mind as well.