I use Microsoft Word's comment feature (Insert menu → Comment) frequently, both when I'm revising other people's work and when I'm working on something of my own and want to come back to it later. Annoyingly, in Word 2008 (the version I'm using) and presumably in Office 2011 as well, there's no keyboard shortcut listed. I finally did some research, and there is indeed a Mac shortcut for inserting a comment!
In Microsoft Office 2008 for Mac, the keyboard shortcut to insert a comment is: Command-Option-A (⌘-⌥-A). Why? I have no idea. On Windows, the insert comment shortcut is Ctrl+Alt+M, which makes just as little sense. Hope that helps!