sujay85 on "[Plugin: WordPress SEO by Yoast] Meta keywords template"

Hi,
I know meta keywords is unnecessary for search engines but I like them for some other reasons.
Now I want to use Tags and Categories of a particular post as meta keywords. So, I had set the Meta keywords template as %%category%%%%tag%%.
Is that OK? I think this is not working on my site. :(

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