With the release of Sitemapper 6.x, the default locations where themes are stored has been changed as well as the format of the themes. This means all themes created with Sitemapper 5.0 or lower are no longer compatible with version 6.x or higher.
While your first instinct will be to scream "unfair", this is actually a good thing. With version 6.x, we have made it so all themes are now responsive, which means they will look great on any device that accesses your sitemap (tablet, mobile phone, desktop). To make all that work, the themes had to be recoded to support this new format. Head over to http://www.coffeecup.com/store/themes to check them all out.