Changing Meta Description & Keywords

I’ve changed my meta titles on a per post basis using a plugin called WordPress SEO by Yoast, but I’m unable to change the descriptions and keywords in the same manner. With the titles, I had to insert a variable in the meta title to reference whatever it is the plugin does to change the titles on a per post basis, but there doesn’t seem to be a variable to place in the meta descriptions and keywords.

Does anyone know of variables I would need to place using this plugin, or if not, something else I can do to change the meta descriptions and keywords on a per post basis? Ever since I went from using a static html site to using WordPress I dropped from the top site for my major keywords to completely off the map and I believe it has something to do with all my pages having the same descriptions and keywords. Thanks!