I have a few friends who use Macs, and the X11 requirement seems to be a real tripping point for them when installing Inkscape. One friend figured it out and wrote up the following instructions:

"If you are running later than OS X Tiger 10.4 you will need to download the X11 software from Apple at http://www.apple.com/support/downloads/x11formacosx.html. Or, if you already have OS X Tiger, you will need to insert your OS Disc #1 and run a Spotlight search in the CD for "X11User.pkg". Once you find and install that file, start another search for "X11SDK.pkg" and install that. Once you have both of those installed (with the OS Disc inside your drive), take out the disc, run the inkscape.dmg install file, and X11 will pop up. It might take a minute or two the first time you try, but it will load."

I am not a Mac user, so I can't evaluate these directions myself. Perhaps someone using a Mac could verify them and put them on the website where folks downloading the .dmg will see it. Making this information more readily to Mac users should reduce the "I can't get Inkscape to work" troubles they seem to have.