To reach Manado from Europe there are several options to choose from. The fastest way is to fly with Singapore airlines to Singapore and then with it's regional airline Silkair to Manado. The same is possible with Garuda Airways through Jakarta or Bali. You could also fly with any European, Middle east, or Asian airline to Jakarta or Bali and choose another domestic airline to reach Manado.
Manado's Sam Ratulangi Airport is well connected with Jakarta, Surabaya, Denpasar, Makassar, Balikpapan, Gorontalo and several east Indonesian cities. Airlines operating these routes are Garuda Airlines, Lion Air, Batik Air, Merpati and Sriwijaya air. If you want to book domestic tickets outside Indonesia try ticketindonesia, because often airlines do not accept foreign creditcards for online bookings.