Menu Admin Bar: "plugin" voice missing… why?

Hello everyone,
I have a problem which I cannot solve.
Last november I bought the domain and the website on WP
https://baglioridicoscienza.wordpress.com/ (I’ve also bought the premium theme for 80 dollars!)
but on the administrator menu bar there is no “plugin” voice.

The matter is:
I would like to create some pages where my readers can see the text and listen the mp3 logging in with usename and password, while the other pages of the website are “free”.
I read in various forums that there are some plugins that does that job.
But in my website I cannot install any plugin!

My menu bar is:
– dashboard
– store
– posts
– media
– links
– pages
– comments
– feedback
– appearance
– users
– tools
– settings

Does anyone have an idea of what’s wrong?

I have another website http://www.laguidainteriore.it/ and there the manu bar is different! The plugin is there…
I really cannot undestand! Please help!

(sorry for my tecnical english)
Thank you in advance!
RobyZ