The mono runtime is dual licensed, GPLv2 and to Xamarin. With the GPL licensed version you have, you legally can't bundle the runtime (even if it's technically possible). By buying Xamarin.Mac, you mostly buy the right to a GPL free version of the runtime, which you can embed in your app.
You can still develop and distribute an app with MonoMac, but your users will need to install mono (through an installer of some sort), and you won't be able to publish to the App Store.
See also Where is the difference between Xamarin.Mac and Mono:OSX