After opening the plugin settings page your Sitemap file will be created automatically.
If you want a Sitemap file to be added to Google Webmaster Tools, just follow the brief instruction. In another case, you should authorize via your Google Account and choose the necessary action.
In order to add a path to your sitemap file in "robots.txt", you do not need to authorize via your Google Account, you should just select the necessary field and click "Save Changes" button. If you're using multisite, the plugin does not allow to add a sitemap to "robots.txt".