LinkWithin has changed the way we install their Related posts widget to a Blogger.com blog. This is how you should do it. First, go to LinkWithin website. Copy-paste your blog address/url into the relevant field and select how many related posts you want to display (see screenshot below):
Then click GET WIDGET! Once you have done this, the LinkWithin gadget will be added to the list of gadgets for your Blogger.com blog.
After clicking GET WIDGET! a page will popup showing the LinkWithin gadget in your Dashboard:
Add LinkWith gadget in Blogger Dashboard
Add LinkWith gadget in Blogger Dashboard
Now click ADD WIDGET, and the LinkWithin widget will be added to the list of gadgets in your blog LAYOUT, probably in your sidebar.
Now hover your mouse cursor over the LinkWithin gadget in the LAYOUT till a cross-hair appears. Click and drag the widget into whatever position you desired, recommended is in the BLOG POST section in the middle, below the post: