In order to add your site to Google Webmaster Tools, you need to get authorized first.
For that, find "Google Webmaster Tools" option on the plugin settings page, click "Get Authorization Code", choose your account, and click "Allow". Afterwards, you will see the authorization code that you need to copy and paste to the correspondent field on the plugin settings page. Finally, click "Authorize" button.
Then you should click "Add" button next to the "Manage Website with Google Webmaster Tools" option on the plugin settings page. After that, your site will be added to Google Webmaster Tools and verified. To get more information on how to add your website to Google Webmaster Tools, check the Sitemap - User Guide.