Check the layout options for twenty eleven…
Appearance > Theme Options
…and make sure the “One-column, no sidebar” layout option is not selected.
‘Content on left’ is selected. I attempted ‘content on right’ but that didn’t seem to make any difference.
Go to Pages > All Pages > select a page for editing, and from the drop down box on the right, change the page template from “Default Template” to “Sidebar Template” update the changes and then retest the page in your browser.
I think that should give you a sidebar in the page. If it does, you will have to apply that template to every page you created where you would like to have a sidebar displayed next to your content.