Set settings for the device, especially OS, Density, Height and Width. Typically it’s something like /Users/yourname/Library/Developer/Xamarin/android-sdk-macosx You could find the SDK directory in Visual Studio at Tools -> Android SDK Manager -> Locations. You might want to keep it in the Android SDK directory somewhere.
Here is a short description how to add them:
Some vendors like Samsung provide skins for Android so you could use them in your emulator when building Xamarin Apps with Visual Studio for Mac.