This will allow you to select any round from 11-99. Turn on the game, then on the title screen, select Action Mode then press Start. Set it to Round 10, set speed to High, and then Music off. Then on Controller 2, press L, R, Select, and Start. If you hear a sound like "Yoshi," you'll know it worked.
How to Unlock a Harder GameAdded 18 Jul 2009, ID #71
This will unlock harder characters in VS mode. To do this, set the mode as COM Then on Controller one, press L, R, X, and Start. If you hear a noise, you'll know it worked. A lot of things have changed just so you know.