The "mint" is peppermint. It is soothing to the stomach.
If you are going to go strictly "by the book", then lax tea and peppermint are all that Burroughs suggests...
BUT some other MC'ers, including myself, have added other uncaffeinated herbal teas without any ill effects. I've personally used Yogi's DeTox tea and Ginger tea in addition to the peppermint.
Check out Zoebess's blog " The Master Cleanse Express". She lists the different teas that she drank during her last Master-Cleanse on there.