It's worth bearing in mind that even though Macau has it's own local currency, Patacas, Hong Kong Dollars are widely accepted and extensively used. ATMs will offer a choice of patacas or dollars and some may even offer RMB (Chinese yuan) although you may struggle to use these while in Macau.
It is quite normal that should you pay in dollars, you will receive patacas in return. When doing this, the shop owner is basically charging you a small exchange fee so if you want to make your money stretch that little bit further, change your money to patacas before you hit the markets.
Changing to or from patacas outside of Macau is almost impossible (even in Hong Kong!) so either spend your money or change it back to RMB / HKD before you leave. If you want to have money to spend when you arrive, bring dollars.
Visa and MasterCard credit cards are widely accepted but a minimum spend is generally required- normally around $100.