What you need to do is go to "plug-ins" and make sure it's activated. Then go to "settings" under all-in-one and make sure it's enabled. You have to make sure it's both activated and enabled.
A good article to show you how to set up the plug-in is here: http://www.wpbeginner.com/plugins/users-guide-for-all-in-one-seo-pack/
If you would like to see if it is indeed working, you can go to this site, which will analyze your meta tags to see what the plug-in is putting out and what it will look like in search engines: http://www.seocentro.com/tools/search-engines/metatag-analyzer.html
Finally, the plug-in is decent, but there are better ones out there now. I understand that the author of the plug-in spends a lot of time developing and maintaining it, but the support is non-existant for the free version because he wants you to pay for the pro version. I personally, along with many others, think this is a little unethical. Either support the free version and earn your donations and paid upgrades, or do not provide a free version.
The site that I referred you to as to how to set-up this plug-in is switching over to WordPress SEO all in one from Yoast and I would suggest that you check it out yourself.
Hope this helps!