Before you can use the Weather Power-Up, you need to first add a location to the Power-Up. On the top navigation bar, click the 'Power-Ups' link. Then click on the 'Get Started' button under Weather

Next, click the blue + Add Location button.

Enter in the latitude and longitude of your location separated by a comma, then name that location. You can use a website like this one to help you find your location's latitude and longitude.

Then click the Create button to save your new location. Once you have a location saved, you can move on to the next step.