you need to add other item into wp-admin->appearance->widgets->right sidebar. if empty, it will use the default such as tag, pages and recent posts for theme.
I like the look of Mesocolumn a lot, but I’m also having an odd time with widgets. I’ve used them to insert ads, and these appear fine, but there is a large gap between the bottom of the sidebar ad, which is in the “tabbed sidebar” widget, and the default items in the regular sidebar. When I insert a widget into the regular sidebar, the ads simply disappear . .
Hope you can help.