To show your iOS device's screen, and broadcast its sound into Ecamm Live, follow these steps:
Connect the iPhone or iPad to the Mac over USB.
If asked, follow the prompts for the device and Mac to trust each other.
If this is the first time, disconnect and reconnect the iPad/iPhone after granting the Trust message.
The device's screen will automatically show up as a camera source in Ecamm Live's Camera menu. The device's sound will automatically come into the broadcast too, and you can control the volume in the Sound Levels window.
If you don't immediately see the device appear as a camera in Ecamm Live's Camera menu or the camera switcher, it may need to be paired. Follow these steps to pair it. Also try unplugging and replugging the device into the Mac.