Martinez is stationed near a radio tower on the rooftop of a blue building, near the Hillside Housing supply point. Climb to the roof via a ground floor entrance on the east side of the building. There are a couple gang members defending it, but it is nothing a few grenades cannot handle. Afterward, climb the staircase and turn left at the top. There are a few enemies on the next level. Stand near the top of the staircase so you can retreat back down for cover if needed. The staircase on the right leads to Martinez. If you are lucky, he may walk down himself, thus saving you the trouble. If Martinez does not pay you a visit, just climb that staircase to the top and take him down with whatever means necessary.