To use an iHealth device with your myHealth Hub app, you must first download the "iHealth Next" app from the app store. In that app, you will see FAQs for devices, including troubleshooting articles.
Make sure your iHealth device is set up per the instructions that came with it, and fully charged. Then, your iPhone or iPad should be able to locate the iHealth device, via Bluetooth settings. Make sure your device (iPhone, iPad) has bluetooth turned on. Do this via your device's Settings.
When you log into your myHealth Hub app, some widgets (i.e., data module types, for example "Oxygen Saturation") will display the blue words, "Connect an iHealth Device" after you tap the plus + button at top right of the widget.
Tap "Connect to an iHealth Device". The app will scan to find your device. If the device is recognized, it will begin recording your data. Tap "Save" in your myHealth Hub app to save the data collected.
NOTE: if your iHealth device was previously paired with another phone or other device, you may have to unpair it there first before you can pair it with the device you're currently using.
HINT: Sometimes you must disconnect your iHealth device from its charger before the app will recognize it.