The nearest airport is in Tambor. There are flights a few times a day by Naturair and Sansa. From here you can choose to wait for the bus from Paquera(Ferry) or take a taxi 20 km to Montezuma.

Take the bus from San José to Puntarenas, and there the ferry
to Paquera. The bus to Tambor, Cóbano and Montezuma waits for
passengers near the ferry dock.

Drive the Interamericana highway to Puntarenas, take the ferry
to Paquera and take the road to Tambor, Cóbano and Montezuma.
The last part between Cóbano and Montezuma is sometimes difficult
in the rainy season - 4WD is recommended.