You'll need to edit one of the files for the theme you're using, and add the following...
Code:
<script src="http://www.wowhead.com/widgets/power.js"></script>
I believe I added that into the header file - but you'll definitely want to put that in a file that will be 'called' every single time (which is what makes header or footer good options here).
Then to get the pop ups to appear, you'll simply need to link to them in your post. Here's a recent example from one of mine...
Code:
<a href="http://www.wowhead.com/?item=39623">Heroes' Scourgeborne Chestguard</a>
If it's still not playing nice, then there's probably a conflict of some sort between that line of javascript and some other line of script that your page is already calling. Diagnosing and fixing that though is beyond my abilities.