Check this out. I don't have 2011 for mac but this looks like it will work.

http://www.ehow.com/how_12094954_rem...grayscale.html

this site has a picture of the menu (scroll to bottom)
http://sustainability.utoronto.ca/pr...ipleslides.htm

I know there is a way to view slides in grayscale or black and white in the windows versions, if that doesn't help you I may be able to dig something up on the view options that could fix it. Is it the output view for the slides that is grayscale?