Step Step Revolution(SSR) Mode:
Once unlocked press Select in the title screen or in the mode selection screen.

Remix Mode:
Once unlocked press Select 2 times in the title screen or in the mode selection screen.

More Music:
Keep playing the Diet mode for a few times then exit the game and you'll find new information. Do this several times to have even more music.
* Tip: You can play this music in nonstop and remix mode.

SSR Mode (for new player or without saving the game):
Turn on the power & wait until the screen say 'Dance Dance Revolution 3rd Mix' & quickly press: Left, Left, Left, Right, Right, Right, Left, Right.

Remix Mode:
After you have accessed SSR Mode, Press: Left, Left, Right, Right.

Thanks to Revolution readers NH and Ian!