Great, you gotta reunite Milli Vanilli, a dream come true :)
If you want to make it a bit easier for you, get the potions north of the stage and the one south of the music store. Open every chest on the island, and go to the music store to buy the CD. Now talk to Fabrice Morvan (the guy with the ducks) but you'll find out that he won't play. Bummer. Now go through the hole on the screen north-west of the stage and you'll see Rob Pilatus try to jump of the cliff. Try to convince him to play tonight, but he won't join either because noone loves him. Go back to the girl near the stage and tell her Rob's situation. Tell Rob now that a girl loves him and he won't jump and play tonight. Now go to Fabrice and tell him that Rob will play, Rob will come, they make up and go to the stage together. Go there too and play the Cd (use your magic button with the Cd equiped) and listen to that lovely music :) End of the D-mod.