Keywords in WordPress

Hello

I have seen in some manuals and other resources in SEO that using a metatag with the keywords is an important asset to ensure a good position in the search engines.

Looking to the code of my blog, and although I am using a plugin to optimize SEO, I cannot see this metatag. I am looking for something like <meta name=”keywords” content=”xxxxx”> in the head.

Can anybody explain me why this line doesn’t appear?
Can it be introduced automatically? And manually? How?