Showing comments when presenting in Keynote
You cannot (if you want to be sure of it, simply search for “note” in Keynote’s help).

Comments are explicitly made for the presenter, not the audience. They behave just like Presenter Notes, except they are positioned on the slides instead of being meta-data.

If you want to show post-it notes on the screen, you'll have to obtain a post-it image somewhere, and then fill it with text.

The easiest way to do so is to take a screen capture of a dashboard widget. I'll be nice, here it is for you  ;)



You can then simply add it as background image of a rectangle, and fill it with text however you please.