Missing Plugins Tab

I’m trying to install a plugin on a client’s WordPress site. I did not set this site up, and I’m not sure if it’s hosted by WordPress or not. My client is using her own domain name, so I’m not sure. Here is a screenshot of what I’m seeing on this site:
https://oi41.tinypic.com/6jdc1t.jpg

And a screenshot of what I see on the WordPress blogs I’ve set up, usually I purchase the domain through GoDaddy.com & install WordPress that way:
https://oi44.tinypic.com/1z69wqv.jpg