This bonus stage comes after Stage 6, where you and Mr. Chorking sing "Rock'n Dreamin'" (a variation of the game's opening song). It's a speedier version, and minus the back-up singers. It's better than the intro version, and lots of fun to play! Also, a few English phrases are thrown in. To get there, you'll need to do really well on Stage 6 by getting a 70+ score from each individual judge, instead of getting the 70 average you normally have to achieve. After you do that, there is a new cut scene (you and Mr. Chorking on stage celebrating your awesome performance, instead of waking up from a dream) and the song will load.
This stage is frantic, but a little more forgiving than the others in terms of scores and "Wow"s and "Boo"s. Provided you complete this song with a passing score, you then get to see a "real" ending cut-scene, where the main character is holding a press conference or something like that after the concert, as opposed to the "she wakes up and realizes it was all a dream" normal ending. Then the game credits scroll by (with a cut-scene set to the original "Rock'n Ecology").
Quick advice on playing this song: unlike the other stages, it's nearly impossible to memorize the lines during the guide's turn, as a lot of those lines are three measures. Just take notice of the first two or three notes, and try to do the rest on-the-fly. And be careful – after all those three-measure lines, don't miss your turn on the one-measure lines! Even completing this song with a perfect score (500 points), there's no special reward, so loosen up and have fun here.
There's no way to re-access the bonus stage without playing Stage 6 again.