Mac OS X:
Pixel Gravity is a Windows program, but there's also a mostly-functional Wine-based version for Macs.
To run it, download and install Winebottler (54 MB) first (unless you already have Wine installed). Once that's installed,
download the Mac demo, unzip it, and run it. It should create a folder called "Pixel Gravity Demo" in your main user folder, which contains its simulations and other files. The full version works the same way (its folder is "Pixel Gravity").
To "uninstall" it and "install" a new version, go to Library -> Application Support -> Wine -> Prefixes and delete the folder "Pixel Gravity Demo" (or "Pixel Gravity") there. Then run the file for the new version that you downloaded, and it should update itself.
Using Wine to run Pixel Gravity requires a Mac desktop or laptop (not an iPad) with an Intel processor (most existing Macs have one, but if you're not sure, here's how to check). Wine isn't perfect, so make sure to try the demo to make sure it works before you buy it!
If you already have Windows installed on your Mac with Parallels or Boot Camp, download the Windows demo of Pixel Gravity and run it in Windows; it should work better that way.