Most travellers reach Masohi from Ambon. Fast boats run from Tulehu, on Ambon Island, to Amahai, the port just south of Masohi. From Amahai it is a short road transfer into town.
You can also travel overland from Ambon by taking the ferry from Liang to Kairatu on Seram, then continuing east on the Trans-Seram road. This is slower but gives a better sense of the island.