Adding sidebars to pages

I have the site default set to no side bar but how do I add one to say the post page. There doesn't seem to be any option per page to turn on the side bar at all. I can select a side bar say 'Optional Sidebar 3" instead of default, but no sidebar appears even though there is something in that sidebar. Also: On my post page http://test.shields.ca/post-archives/ Only one category is appearing. I think they were all there at one time, but for the life of me I can't find where I changed that setting.