Every route to the Mentawai runs through Minangkabau International (PDG), on the coast of West Sumatra. Jakarta feeds it all day, Kuala Lumpur three times daily, Medan once.
The boats sail in the morning, so give yourself an evening in Padang first, with good food, an easy night, and a short drive to the harbour at sunrise.
Tell us your flight and we take it from there: the airport pickup, and the car to the port in the morning.