The Application Links plugin is bundled and shipped with the Atlassian applications. You cannot install it yourself.
The guide you're reading is for administrators who want to configure application links between Atlassian applications. The guide contains information on:
Having trouble integrating your Atlassian products with Application Links?
We've developed a guide to troubleshooting Application Links, to help you out. Take a look if you need a hand getting around any errors or roadblocks with setting up Application Links.
Developer resources are for those who want to develop with the Application Links plugin. Take a look at the Application Links documentation on the Atlassian Developers site.