Finally, after some web searching, I stumbled upon an answer. Using "feeds/posts/full" as the address for the sitemap seemed to work.
So, if you are trying to get the sitemap to work in Webmaster Tools, follow these steps:
1) Log into your Google Webmaster Tools account (http://www.gooogle.com/webmasters/tools).
2) Click on your site--we are assuming you already created your account and verified the site
3) Click on Sitemaps under Site Configuration
4) Click on Submit a Sitemap
5) At the time of this post, doing the above shows a box that has the base web address filled in and an empty text field to add the location of the sitemap. In this empty text field, add feeds/posts/full and click Submit Sitemap.
That's it! It should show up shortly. Check to make sure there are no crawl errors and you are done.