Go to your Dash board, then Settings, Click Other, Then Set "Allow Blog Feeds" to "Full"
And now, to add your site map, this all you need to do:
1. Go to Google Webmasters Website
2. Select the site which you would want to add a sitemap.
3. Click Site Configuration, Then Sitemap
4. Click "Add/Test Sitemap" on the right.
5. Copy and paste this: atom.xml?redirect=false
6. Refresh the page and your done.