Obviously, you can only do this in the Rocky Mountains destination. When a customer needs help with the sightseeing binoculars, look around the scenery for Bigfoot instead of looking where the indicator tells you. When you spot Bigfoot, keep the viewfinder on him for about a second, and the Achievement will pop. It will also satisfy the customer. DO NOT read the last paragraph of this guide if you want to try to find Bigfoot on your own. As far as I know, Bigfoot will never be a sightseeing target. You have to find him when the customer is actually trying to look at something else. Once you've found him, looking at him again in a different instance will not do you any good.
[SPOILER]Bigfoot is hidden near the lower-left of the scene. You can make out his silhouette at the treeline close to the lake shore, directly left of the kayakers. He's in the shadows of the trees, so his image is very dark. He's in that famous mid-stride pose from that old supposed actual photograph that you can see on Wikipedia's Bigfoot page.[/SPOILER]