You can install the add-in via multiple ways:
Open the Store
The most convenient way is to simply open Outlook Desktop app and open the Store via the button in the top toolbar.
Install the Add-in
In the Store search for "OTJ" and you'll find OTJ Jira for Outlook.
Click on Add
OTJ Jira for Outlook menu will be available in Outlook Ribbon
That's all! Now you can simply Login and Start Enjoying OTJ Jira for Outlook.
The most convenient way is to simply open Outlook in a web browser (IE, Chrome, Safari, Firefox, etc...).
(Note: You can also use this link directly from the Microsoft AppSource to Get OTJ JIRA for Outlook.)