Go to Juni’s Secret story (see the Knytt Stories trophy) and start the quest. Jump over her house on the left and drop all the way down. You’ll get to a small cave right under the house. In the middle of it there's a small corner sticking out. Grab it and move up for a hidden path. Follow the path to get to the disco area.
Credit to: http://www.ps3trophies.org/game/knytt-underground/guide/
Credit to Mr Vuule for the video