The Magenta Den is located in the Highland Field and is the first dungeon in the game, serving as a tutorial dungeon to help familiarize you with dungeon "physics". To find it, build the bridge up the ledge near the landing site that leads to the spores and the Burgeoning Spiderworts. There's a rock formation nearby here that looks out of place. Have your Pikmin swarm it and the dungeon entrance will appear! It's the beginning dungeon, and has only three floors- no hazards, and not even a final-floor boss to do battle with (don't worry, you'll have plenty of those later). What it does have, however, is a patch of Violet Candypop Buds on the final floor so you can add Purple Pikmin to your team! There are a couple of treasures and weak enemies to be found here as well.