These Installation instructions explain what to do when you encounter the "Untrusted Enterprise Developer" message on iOS devices.
If you haven't downloaded an app before, and it isn't in the App Store, you'll be prompted to go through a few steps to trust the app.
First, tap Cancel to dismiss the message below.
Next, tap Settings > General
Then, Device Management
Now, tap the app certificate in the ENTERPRISE APP section.
Finally, tap "UserTesting Inc", then tap Trust again to complete the process.
Now you're ready to start testing!