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Did you unzip it into "C:\Program Files (x86)\Adobe\Adobe Photoshop CS3\Plug-Ins" or equivalent?

(By the way, it almost definitely won't work with CS4 64-bit, since it's a 32-bit binary.)

I hit a crash when using it in CMYK mode while a single channel was selected. I'll fix that at some point (though I doubt anyone but me would try using it in that mode anyway).