Simply search "Grand Theft Auto China Town Wars for R4 Card" on google. Make sure that your R4 card is running the latest firmware. If it isn't, then access www.r4ids.com and download the latest firmware.
@Xdqwerty, Osu! Tatakae! Ouendan! is the Japanese version of the game, different settings/characters/songs but otherwise identical mechanics. I played that before I knew about Elite Beat Agents lol. Both fantastic games https://en.wikipedia.org/wiki/Osu!_Tatakae!_Ouendan