When using Tier 1 your program is created using a Windows IDE. If you want to test this on different platforms you can broadcast your game to the AppGameKit Player. It's a useful way of being able to test your games on different platforms.
Distribution is handled a little differently dependent on which platform you are targeting. You will likely end up bundling the AppGameKit Player and your code and media all in one application, which you can distribute to end users.
Currently Mac developers can only use Tier 2, which means you'll be creating your game in Xcode using C++.