This page contains usage instructions for Mockups for JIRA.
The main Balsamiq Mockups help page is here.
JIRA Administrators: the Mockups for JIRA Admin Guide is for you.
To add a UI Mockup to a particular JIRA issue you simply click on the “Add/Edit UI Mockup to this issue” link:

You will be taken to the Mockups editor. When you are happy with your mockup, you simply go to the Mockup menu:

and select “Save and Close” (also note the “Save as New Mockup” menu item, which is exclusive to Mockups for JIRA. More on that below).
You will be prompted to name your mockup, and the software will auto-generate a simple date-based name for you, which is handy when you have many mockups on the same issue:

You can just hit enter to accept that name or give it whatever name you wish.
You will be taken back to the issue. Here's a sample one (click for a larger view):
Notice how the UI Mockup is now attached to the issue, both as an image and as a bmml data file (BMML is the flavor of XML that Mockups uses). Also notice how Mockups for JIRA has added a comment to the issue for you, saying “Added UI Mockup: <mockup name>“. This means that if any of your co-workers is watching the issue, they’ll get an email notification to check out your new work of art. :)
So that’s the basic workflow of creating a new mockup for an issue. It gets better!
Say you want to tweak an existing UI mockup or create a new mockup as a variation of an existing one. The workflow is simple. Click the “Add/Edit UI Mockup to this issue” link again:

This time you will be presented with a dialog like this one:

You will then be able to either start from a blank canvas or edit an existing Mockup from the ones already attached to the current issue.
To simply iterate on the same mockup, you just select “Save and Close” from the Mockups menu, to create a new mockup variation, you select “Save as New Mockup” instead.

If you select “Save and Close”, you’ll have a chance to enter an optional comment about your edit:

As the dialog says, if you leave the field blank no notifications will be sent. Otherwise your comment will appear on the Issue’s comments list, and watchers will be notified.
That’s it! Couldn’t be simpler right? :)