Harvest The Jungle is the third main quest in Far Cry 3
How to Start
This quest starts after you open your world map and set the Boar territory as your waypoint.
Learn more important mechanics in the game such as hunting and crafting, and fast travelling
First, while in the world map, move the cursor to the Boar Region and choose it as your waypoint. Next, use a white car near you, the Darrah, to get there. Then, exit the car and start your hunting. You should find all the leaves you need, as well as the Boars, in the hunting grounds. See here if you are having trouble finding any leaves or finding Boars. Drive back to Dennis and talk to him, he will walk you through crafting some items and choosing new skills, Then, board the Technical to complete this mission.
"You may even find more powerful plants out in the woods, if you are lucky" (Dennis)
"Use the skins of the animals you killed to enlarge what you can carry. Crafting will allow you to carry more weapons, money, loot, and syringes" (Dennis)