As of May 15, 2018 WeatherUnderground no longer provides free API keys. Any existing API keys you have for WeatherUnderground will continue to work.
Go to http://weatherunderground.com, click join in the upper left (or login if you already have an account), enter an email address and password and click "Sign up for free".
Once logged in, at the bottom of the page, click the link for "Weather API"
Next click on the "Pricing" tab, make sure the "Status Plan" is selected and click "Purchase Key"
Now fill out the required fields in the form and click "Purchase Key".
On the next screen, highlight and copy the "Key ID".
Lastly, paste this into the "API Key" field in the DAKboard Weather options and click "Save Changes"