I think this is what you do:

A just plays the "song"
S is the actual game.

When it plays through the group the second time, you're suppose to hit the note that's marked with an "X" or a cross(given this is Rumia), using ZXC
Z is the high note
X is the middle
and C is the lower.

It might be easier to play the game by reading what's under the staff after getting used to that though.