Talk to her first, then when it says something about opening the menu with Start and assigning it to the C buttons, just show her the potion(just push the button it's assigned to while that text is onscreen.).
i have talked to her and i have z targeted her. i have done everything right. i have mastered ocarina of time so i know all the ways of how the game works. it really seems like the game has glitched out because i have tried everything