I don't see anything in the documentation on how to use this feature. When I try to add a milestone, it gives me an issue creation screen. Not sure where to go from there.
referring to my latest version (pre-v2.2, available for downloading on my server see below), you have various possibilities to create a milestone having enabled re-scheduling as prerequisite:
On the Gantt-Chart, a milestone is visualized as rombus/carrée in green or red (if planned end date is later than due date) on the issue's row and in white-gray on the issue's grouping element to focus attension on.
My early-adapter-program (EAP) for download of current pre-release 2.2 depending on your JIRA version:
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