First of all, find a monkey. Follow the monkey, and eat everything the monkey eats. If possible, eat the monkey too.
I've seen capuchin monkeys eat the fruit of Strychnos, the genus that strychnine comes from. That could be a fairly risky thing for a human being to consume, especially if you happen to eat any of the seeds, then all bets are off.
Many monkey species, especially those that eat leaves (these would be the most common species you'd encounter, at least in the new world) walk a very fine line between eating enough of a given plant to satisfy their energy budget and eating too much and poisoning themselves. A human following their example is likely to end of with a stomach full of undigestible fiber, get sick, or worse. Energetically, you'd probably spend more calories following the monkeys around than you'd gain from any food you'd collect.
Now eating the monkeys, that might be a good idea, but the primates you'd most likely encounter would be in social groups, and once you kill one of them, it will be very hard to kill another from the same group.