Here is the schedule I have from last year. It is also posted at the bus depot in Melaque.

La Manzanilla Bus Schedule

Departs La Manzanilla: 8:30 & 10:30 a.m. and 1:30, 3:30 and 5:30 p.m. The bus originates just south of the primary school. Many people board at the corner of Cafe del Mar but you can wave it down anywhere along the main street and it will stop to pick you up.

Departs Melaque: 7:40 & 9:40 a.m. and 12:40, 2:40 and 4:40 p.m. The bus station in Melaque is located near the middle of the main street going through town.