Not sure, but it might be possible to install the .net framework on OS X. That's just from a quick Google search, and I don't truly understand this stuff, but take a look. I believe you can also use Wine to run it.
Even though this isn't directed at your use case, take a look at this post about alternatives. This post also suggests alternatives, including Pixen, PikoPixel, Piskel, Aesprite, and setting up GIMP to work well with Pixel art. Forewarning, I don't do much pixel art (I've always just used GIMP) and therefore can't give you anything other than other's advice.