As of May, 2019 WUnderground only offers free access to their service for owners of weather stations that push data to their system.

To use WUnderground with RainMachine, weather station owners need to get their WUnderground service free key from their member area.

We will continue to improve and research other ways to offer observed data to users without a weather station.

Thank you for your understanding,
RainMachine Support

EDIT June 20, 2019
While WUnderground free access might still work we don't recommend using WUnderground service without an API key.
Thank you!