On the official forums, it was mentioned that Pixel Gun World isn't coming back which means it's now impossible to even try to run it on the Mac platform. There is a version of Pixel Gun called Pixel Gun 3D on the iPad and iPhone, but that's pretty much it.
If you want to try, you can download the Windows installer from the application's page at Software Informer and then use Wine Bottler to emulate it on the Mac OSX.
