How do I add the Evernote Gadget to my JIRA dashboard?

Simply go to the desired dashboard you'd like to add the gadget to and click the "Add Gadget" button in the top-right of your dashboard.

In the resulting window do a search for the term "Evernote" and then click the "Add gadget" button next to the Evernote Embed Gadget listing.

How do I populate the Link text field with my Evernote note?

After installing the gadget you'll be prompted with these options:

To get the proper link head over to the Evernote web application (make sure to use the web application and not the desktop client).

Navigate to the note you'd like to embed and click on the down arrow next to the "Share" button in the top-right. Then select the "Link" option:

In the resulting overlay copy the URL and paste this into your JIRA gadget!