You get the code from the Powered by Wowhead page.
<script src="http://static.wowhead.com/widgets/power.js"></script>After copying the line, open your blog's admin section and get to the part where you can edit the blog's source code directly. In Blogger's case that's located under Layout > Edit HTML.
Paste the code right under the "head" tag. You are done. Wowhead's tooltips should work now.