Most travelers arrive from Puerto Varas or Puerto Montt, but it's also possible to take other gravel roads and ferries that arrive from the south - Puelo, Horno Piren, Parque Pumalín. Below are directions based on where you're coming from.
From the north or Bariloche. If you're driving from the north, it's not necessary to drive all the way to Puerto Montt. From the Panamericana, Route 5, get off at the Puerto Varas exit. Get on the road along the coast heading southeast and eventually east - Highway 225. If you're coming from Bariloche, keep in mind that the border closes at 8 p.m. and opens 8 a.m.
From the south. It's also possible to come from the south via the Carretera Austral. The ferry that connects Parque Pumalin to Horno Piren runs only January through February. Otherwise you'll have to take a ferry out of Puerto Aisen or Chalten to Chiloe or Puerto Montt. Check out the Navimag Ferries website for schedules and costs.
To get to Cochamó and the trailhead from Puerto Montt or Puerto Varas, go to Puerto Varas using Highway 5. There are two exits. Either will do. Drive down the hill into downtown and head towards the lake. Get on the coastal road and head southeast. This turns into Highway 225 and goes to Ensenada and Volcán Osorno. After less than an hour you'll pass through Ensenada and the highway will curve south. A few kilometers out of Ensenada you'll arrive to a major junciton. Slight right and head in the direction Ralun-Cochamó. After half an hour, you'll arrive to Ralun and you'll cross the bridge spanning the Río Petrohue. After 10 kilometers, the route turns into a gravel road and narrows as it borders the Estuary Relonquavi's coast for 17 kilometers until you reach the Cochamó town. This is the last place to buy supplies or arrange for pack horses.
Continue past the town three kilometers. The road edges along first the estuary and then parallels the Cochamó River, entering the Cochamó Valley. Just before getting to the bridge that spans the Cocahamó River, turn left (norhteast) on a gravel road and continue eight kilometers nearly to its end. You can opt to park on the road or at Camping Los Pozones for 2,000 pesos (~USD$4) per day. From there walk to the road's end and cross a wood bridge and pass through a gate of which marks the beginning of the trail.