Well, they're probably not what you're expecting...
"Zoom" is the Shrink/Enlarge option (Hold L or R to switch between them) on menu 3. The problem with it is that it destroys your image when you use it; if you shrink and enlarge, you'll notice the fine detail was lost, and if you enlarge and shrink, it'll crop everything that was outside the page.
This is serviceable for drawing small sprites. Draw every sprite on its own page, then shrink it down to the appropriate size. When a sprite is ready, circle it with the select tool, then copy and paste it to the appropriate page of animation. I've seen sample flipnotes where an assortment of sprites have been copied and pasted to a single page to serve as a sort of template.
Again, I don't promise that this is how sprite flipnotes are made; this is just my best guess based on the tools available and how I would try to do it.
To use the "fill" tool, you have to draw the area you want to fill using the select tool, and while the area is selected, blow into the microphone. The area will be filled with the currently selected Brush pattern.
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They do draw it, mostly. Another good tip is to put the sprite on your computer, enlarge it, take a photo of it with your DSi and then import it to Flipnote. You will then need to clean it up a little.