Late disc 2 or disc 3 are the only times it will work, and by work, I mean give you her ultimate limit break, "Final Heaven".

The notes you need to play are as follows:

"Do Re Mi Ti La Do Re Me So Fa Do Re Do"

When it ask you to choose "I forgot" or "I remember" just choose "I forgot" so it will show you all the key directions to what buttons play what notes.

And yeah thats how you do it.