I have difficulty giving directions, but here goes:
In Melaque you take the main drag, with the Dolphins, west to the last street. Make a left by Intercam Bank.
Go about two blocks and before you go three blocks, the place is on the left. Gated courtyard, I believe with small, not working, fountain or statue. A guard is there that can direct you.
This place houses Melaque government offices/services. You go to the back of the patio. The immigration office is the second door on the left, across from sanitarios and there are signs. Very nice folks.
I am headed to Cihuatlan tomorrow AM. You can join me or follow and I will drop you off. 315 108 9280, Cruz