can't add plug in to Yoko theme

I want to add a form (purchased from Gravity Forms) to my blog so that people can contact me regarding work via the form on my hire me page.

I bought the form and tried to download onto my site however, when I try to follow the instruction on how to add a plug in I am supposed to find ‘plug in’ under the Appearance title in my directory. It is not there.

I wondered if it was because Yoko was an old theme. I like Yoko because it is clean looking however there was a warning at the top of the page (which I paid no attention to at the time) that said Yoko hadn’t been updated for 2 years.

Am I doing something wrong or will I have to change my theme?

my web site address is unhurriedchange.com

many thanks in anticipation, kind regards, Vivienne