Personally I don't think that drive will be something you can do right away. Like in Kh1 and magic you went a whole world level before you got it. But the drive meter will be on the display screen, just no of the drive bubbles will drop out of enemies (like the magic bubbles) until you have that ability, I mean think about it the first few Heartless you face you can suddenly go to brave form and beat the hearts (hehe word pun) out of them. And also i think that drive levels (When the bar fills up for the first time a one is seen next to the bar, then when it fills up for a second time a two is next to the bar etc. thoes are drive levels) are abilities that you equipet(sp) like you will learn a drive level 3 and you have to equipt(sp) that ability to beable to raise you drive level to a maximum of three.
Still it also could be that you can build you drive level up as much as you want until you press the 'morph' button (i guess thats what they call it) and then you drive just dropps until you cant pick up any more drive bubbles.