[Plugin: WordPress SEO by Yoast] No Meta description tags in source file.

did this: Added the meta descriptions and keywords to each page.

I expected the plugin to do this: Change the way the snippet under the Title appears in search engines( not immediately but over time). Change the meta description in the html source file of that page.
For example on another site:
<!– This site is optimized with the Yoast WordPress SEO plugin v1.1.5 – http://yoast.com/wordpress/seo/ –>
<meta name=”description” content=”Now learn Kiteboarding, Snow-Kiting and power kiting lessons with us at an affordable rate. Experts are there to get you the best services which will help you to learn the lessons efficiently.”/>

This meta description is added to the html source file. When i try adding descriptions to wordpress page using yoast plugin. No changes occur to the page.

Instead it did this: No change at all.

Any suggestions. Sorry if i am unclear regarding anything and i will try my best to answer questions regarding any misunderstanding.
Thanks a lot.

http://wordpress.org/extend/plugins/wordpress-seo/