Dennis is the Cheapska- i mean, Green Tree Frog that gives you a few missions on Bridge of the River Ty, you meet him a bit later on the level and he'll give you a mission for guiding him to his home by lighting the barrils with your Flamerang, after that, you'll reach Dennis' house, the key location for this achievement, go to the door and interact with his house by pressing a button when the "!" Icon shows up, which will play a message from Dennis, giving you the achievement.
Doing Dennis' mission is not required for this achievement, but it is useful, since it'll give you the location for his house and you get a Thunder Egg for doing so.
There's also a Golden Cog in his house and a few Picture Frames near it, just to let you know.