I'm not a MAC person. My understanding is that most business applications like PowerPoint output a 4:3 or more square like image so I would try the projector on Normal. There should be a proportional image with side bars since you projector has a native 16:9 HD widescreen native aspect ratio.

If you play a widescreen movie on your computer and it adapts the output to a widescreen aspect ratio the your projector should be set for widescreen.

If I'm mistaken smarter folk will help me out and correct me.

EDITED to add that our 720p projector has an "auto" mode and it usually detects the input image aspect ratio and scales or adapts it's output to match that input.
goodluck