There is some stuff on the magic mailing list about using Magic as a GDSII viewer. The problem is that you need to create a technology file for the cells that you are going to view. I don't think you can just view arbitrary GDSII files without setting up the technology file first.
If you are looking for an easy to use GDSII viewer, try StreamVista. It runs on Windows and is available for download at www.semigy.com.