Temperatures are now given in Kelvin and this plugin uses an offset of 273.15 to convert to degree Celsius.
If you're using version 115fb0875f or prior you might consider renaming your symlinks from "weather" to "city" - by default the script will do rename internally for compatibility reasons (SYMLINK_COMPAT flag true).
Clone this repository or download the openweather_ file and create a symlink as root in /etc/munin/plugins by using e.g.:
cd /etc/munin/plugins; ln -s /path/to/openweather_ openweather_<type>_<id>
where is currently ignored (formerly one of [station, weather]) and is a suitable city id. (You can easily get the ID when you navigate to your station/city of interest and open the detail page for this station by clicking on it - your URL should contain the id then.)
Don't forget to restart your munin-node deamon.