You can find and install the plugin directly from within Jira in the Apps section. Just search for the keyword 'aqua'.
You can optionally also download the plugin from the Atlassian store using the following link and then upload it to your Jira environment.
To upload the add-on:
1.     From the application's administration console, click the Manage add-ons link.
2.     Click the Upload add-on link at the top right side of the page.
The following dialog appears.
3.     Enter the location of the JAR file to upload using the file chooser or by specifying a network location by entering a URL.
4.     Click Upload.
A confirmation message appears when the add-on is successfully installed.
5.     If prompted, restart your application to have your change take effect.