I also used posts on page plugin so the post listing was essentially a page and the display widgets plugin allowed me to only add categories on the right hand side of a specific page
just thought i would add that i solved this via a plugin that lets you to select what pages the widgets appear on. the plugin is called display widgets.
Hi I have the same problem did you manage to resolve this on the posts (listing) page the right hand side bar does no appear. It appears fine for other pages and for individual blog posts
