Hi David
Do a global search in the project for the text "ZoomButtons" and you should find the object. I believe it is a custom object created by the AppStudio team, so you can go into it and customize it a little, e.g. change icons etc.
If you want to use the object in your own app, make sure you have this import statement:
import ArcGIS.AppFramework.Runtime.Controls 1.0
then just add the object and go from there...
Cheers,
-Paul