Weather is a premium feature so you will need to first be logged into and have completed a sync with your active informant sync account at informant settings>accounts>informant sync.

Then go to Settings>weather and choose a location or allow automatic to find your location based on your devices location currently. Then choose a 'show in' option to pick the view you want to see the weather option in.

If your weather still doesn't appear, make sure you have a good internet connection on your device and aren't behind a firewall or VPN. Then choose a location instead of allowing automatic.