first off make sure the red dot will even mount. Some of the RED DOTs like a BSA and Simmons(and I do not think the Millet is one of them) will not span the open breech of some muzzleloaders like my Black Diamond or Wolverine. If the mounting screws are after the breech opening like on the break open, that will not be an issue.

Other then that you put the same base (weaver style usually) on your rifle and then mount the red dot to that... I have a couple of rifles with RED DOTs and they work good for close range (75 yards and under) shooting. Be sure and use a little locktite on the screws, the aluminum screws that come with some of the red dots do not hold well.