appearing "next post" widget

I want to include a function like seen in Mashable. It’s the next post widget. If you reach the end of the post, a suggestion will appear on the right side. I found a solution for this http://clokey2k.com/blog/2011/09/08-mashables-cool-next-post-widget/, but it’s not working for me.

Here is how i did:

1, added a nextpost.js file to the root of the webserver

[code moderated - please follow the forum guidelines for posting code]

2, placed the following code in the head:

<script type="text/javascript" src=".../nextpost.js"></script>

3, addded codes:

<div class="next-link">fb-share button</div>
<div class="next-box">code of a wp widget area</div>

The problem is, that the widget area (next-box) is always appears, not only when I reach the next-link section. I think the proble is with the javascript integration. What should I do?