Okay, you're taken to the difficulty select screen. Go to Muzukashii and continue to tap the right kat 10 times. That's to unlock Oni.
Go back to Kantan and tap the left kat 5 times to go to Baizoku. 5 more times gives you Sanbai, another 5 to Yonbai, Doron, Abekobe, and then back to regular mode.
To unlock the secret songs...wait, I bet they have them unlocked already, so you shouldn't need to worry about that. To get to Urahumen, tap the left and right kat alternatively while on the title screen(after slotting in your coin) 20 times. Taiko 11 will display 'Hazure?', and Taiko 12/12.5 will have the logo shine golden, if you do it correctly.
arken: You're making Koibumi 2000? That's another set of my Taiko maps down the drain. With complete lyrics too.

Oh well.