At the bottom of the same page there is a region embedded code. This code should be copied and added to the theme you are using in the header.php file immediately after the opening tag <head>.
Add the following CSS code in the style.css file of your theme. Now Google custom fonts can be used successfully on its website WordPress:
.h1 site-title {font-family: 'Open Sans', Arial, sans-serif;}
If you do not plan to use all the available services to download fonts that are connected to this plugin, I would not advise you to use them to access only to the Google Fonts - there are solutions that are simpler and easier to set up.