To display the button for Google login, you have to enable the option Enable Google Login that you can find in the plugin option panel.
First of all, click on this link: https://console.developers.google.com/ and then click on Credentials and select the O Auth Client ID entry.
Click on Configure consent screen
Enter the required data.
Select Web application and click on Create button.
Please note: Copy the Callback URL value from the settings panel and paste it into Authorized redirect URIs in the plugin settings.