My guess is that you are on JIRA Cloud. The add-on is not available for Cloud.
If my guess is wrong and you're on Server, then yes, you can install it from the marketplace from inside the "find new add-ons" under admin. Or, if your server can't reach the internet, download it to your machine and use "upload add-on" to upload the jar.
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events