After opening the plugin settings page your "sitemap.xml" file will be created automatically.
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.
If you want "sitemap.xml" file to be added to Google Webmaster Tools, find "Manage Website with Google Webmaster Tools" option and click "Add".
In order to add a path to your "sitemap.xml" file in "robots.txt", you should just select the necessary checkbox next to "Robots.txt" and click "Save Changes" button.