Does MakeUseOf have a manual for the use of Excel for Mac, specifically for OS X 10.6?
More important than the OS version would be the Excel version you are currently using.
Assuming you are using the latest Office:Mac 2011 you can actually find a good getting started guide on the Microsoft Office for Mac homepage. Their Help & How-To section also offers some useful stuff.
I think once you manage those basics anything further is on-demand learning and for questions you can always consult the Excel for Mac Answers section.
presently, we don't have an Excel guide at all. However, maybe someone can suggest alternative Excel guides for the Mac. I found this one at TutorialGuide.