Google wants you to verify that you are the owner of the website your domain name points at before it will let you insert the link that SupaDupa gives you for the sitemap URL.
There are a number of methods you can do this by, but as you do not have direct access to the SupaDupa servers, the recommended approach is to verify your Webmaster Tools account via the Google Analytics code that you've entered into your Shop Settings (this is something you'd need to do first!).
Note that in some cases, Google Analytics will already be under the Recommended Method tab
Add Google Analytics to your Boutique first. if you have not already done so go to google.com/analytics, get your analytics tracking code ; then in your SupaDupa Back Office go to Shop Settings => Settings, scroll down to Google Analytics and add the Google Analytics code you have just got.
Next, when you add your site to Google Webmaster Tools, you will be prompted to verify your ownership of the site (your SupaDupa store). From here, select 'Google Analytics' from the 'Alternate Methods' tab. Finally, click 'Verify', and you should be good to go!
If you are having trouble verifying your Domain Name, please ensure that the URL you have added matches the Domain you have set as your Default Domain in your SupaDupa Store backend.
Any problems? Let us know.