One of the two Pacific turtle nesting beaches in Nicaragua, the park at La Flor is managed by the governmental environment agency MARENA.
Foreigners pay $10 per person to enter, another $25 for a group for camping (bring your own gear). Make sure you catch one of the nighttime arribadas, or mass nesting events, that occur during the crescent moon from July to February.
Casa Oro in San Juan del Sur [1] runs shuttles during the season.
Links:
[1] http://www.moon.com/destinations/nicaragua/southwest-coast