Why can't isn't Plugins available as an option??

I’m very new to this, and I had this site set up about 6 months ago and am just now starting to utilize it.

i want to add a pop up window when people first come to my site, and I was told that in order to do that I needed to go to plugins, but plugins is not there to use for me.

I see posts, media, links, pages, comments, grid options, contact, profile, tools and easy social share buttons.

So, can anyone help me?? Why isn’t that an option for me?? I was told if I install mail chimp I would be able to install the popup under plug in, and after I installed mail chimp it still isn’t working.

HELP!