Originally, we were charged with figuring out how to display SSAS cube measure descriptions via ToolTip in Excel 2007. If that's your plan, forget it - after some reading up on the interwebs, it appears that Excel doesn't even request the Description property. Additionally, if you want to add a description to Calculated Members, you have to hack it in (yuck).
So we went with a simple, albeit relatively crude (but effective), alternative - implementing a URL action for Cells so users can easily link out to a definition of the measure they're looking at.
Create a new action in your cube (Open up the cube definition, Actions tab) and configure similar to this:
Name: View Member Definition Action Target Target Type: Cells Target object: All cells Action Content Type: URL Action expression: "http://i.domain.com/doc/Defs.aspx#" + [Measures].CurrentMember.Name Additional Properties Invocation: Interactive Description: View Member Definition "View Definition Of " + [Measures].CurrentMember.Name + "..." Caption is MDX: True