Mipo Odubeko scored a brace while Teden Mengi and substitute Charlie Wellens grabbed the others after Co. Fermanagh came from behind twice.
Teden Mengi scored twice as United started their campaign well in Northern Ireland.
United will play Colina (Chile), Co. Fermanagh (N Ireland) and Southampton (England).
After ending a 25-year association with the tournament in 2015, the Reds are back at the Milk Cup, now known as the SuperCupNI.
Goals from Kyle Wych and Teden Mengi secured a comfortable 3-0 win at stadium:mk in front of 3,000 people.