After you've paired your Pebble with your phone, you can enable it in xDrip+.
xDrip+ will then install itself the watchface (no need to download it).
It will also propose you to install the snooze button app, if you skipped it you can do it later with Install Pebble Apps at the bottom.
You can select different watchface.
You can enable vibration.
On a special value (typically 100 mg/dl or 5.5 mmol/l) you can display a text message (traditionally BAZINGA!).
If you didn't install the snooze button app you can force install it here: