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What worked for me was having CS4 installed (can be trial or activated) when you install cutting master, it'll detect CS4's installed and confirm to drop the plugin in CS4's Plug-ins folder, after it's done browse to the plug-ins folder and copy CutPlot403 to Illustrator CS5's Plug-ins folder and voila. (You can uninstall CS4 after) That worked great for me on Windows and OS X. If you need Illustrator CS4 just download the trial from Adobe. Good luck!

I've been using it fine in CS5 for some time now, I use it on 2 machines, Mac running Snow Leopard and a PC running Windows 7 64-bit both sporting Illustrator CS5 without issue. It just takes a little creative finesse.