From other parts of Indonesia, you can reach Manado by bus, boat or plane. Bus is of course only an option from other parts of Sulawesi, and isn't really recommended. To get to Manado from Makassar or Palu will take several days.
You can reach Manado by boat from other islands using the Pelni line. Most of their ships are rather modern, with first, second, third and economy-class cabins. The journey from other islands may easily take two or three days.
By far the easiest way to get to Manado is by plane, and that is about the only way to get there from other countries. There are [sometimes] direct flights to Manado from Singapore, Kuala Lumpur and Davao, as well as from other parts of Indonesia, such as Makassar. In fact, most flights from further destinations, such as Jakarta or Denpasar, are usually via Makassar. For more information at airlines, flight schedules, and other airport information, see our Manado airport guide at Asia-Airports.com