The manual you linked to seems to be the right set of steps.
The screenshot you posted though indicates you do not have a Pebble connected, which may be the root of the problem. I believe the developer option only appears if a Pebble watch is actually connected. Once its enabled, it will stay that way though.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
