Do you mean in Normal view or in Slide Show view?
In Normal view, it depends if you want it to look good. You can draw using
the Scribble "AutoShape" (AutoShapes>Lines>Scribble). If you have a tablet
with an electronic pen, it might look OK. It might look better to write
something on paper, scan it, and insert it as an image.
In Slide Show view, you can write using the Inking feature. Right-click and
choose one of the Pointer options. It won't look very good, but would look
better if you had some type of electronic pen instead of a mouse. A mouse if
not a very good way to write!
Ellen