Use "inspect element" to view your archive or index page where you're wanting the plugin to work.
The "Content Selector" should be the container (div, section) which wraps all of the posts. (If you're inspecting element with Chrome and you hold your cursor over different parts of the HTML, it will show you what is contained -- you may have to expand a bunch of sections to find it.
The "Navigation Selector" should be the container (but not the "a") for the Next/Previous post links.
The "Next Selector" should be the actual link ("a" element) inside Navigation Selector container.
The "Item Selector" is the container of one individual post.
If the bfBlank theme follows the same scheme for these as the parent, here's what I see for the selectors when I inspect the Big Feature theme:
Content Selector: #content
They do have a little different post navigation setup than some themes with the page display. I believe that link is the right one, though. If it doesn't work, maybe you can copy and paste the contents of your index.php file and I can look more specifically at your setup.