Hi Elliot! We've heard this feedback from a number of customers, and we have an issue tracking it. It's also in UserVoice here: Zoom tool in Prototype Mode – Adobe XD Feedback : Feature Requests & Bugs
Don't worry, we will be adding this!
Great, XD has the potential of being a great app.
Thanks for the reply Stuart!
Good news! You can use the Zoom tool in Prototype mode right now, even though it's not in the toolbar -- press and hold Space, then press and hold Cmd. As long as you keep those keys held down, Zoom mode will be active. (This trick works in Design mode too! And most other Adobe apps.)
Note: if you've just upgraded to 10.12, the new OS-wide keyboard shortcut for Siri will interfere with this. We're working on an alternative shortcut to get around this, but in the meantime you can change System Preferences to either disable Siri or switch it to a different global shortcut.
Hope that helps!
Thanks Peter. Re-training fingers time.