How do I enable and use the Roadmap feature in the Mantis bugtracker?
I am answering this myself and I hope it will be useful to other Mantis users. Here is a discussion on the same topic.
Update: As an example, here is the Mantis Roadmap for Mantis itself.
Answer
From what I understand, you update the project and add one or more versions first. Later, set the target version of issues in the project to one of these values. You can also do this from the "View Issues" page by selecting "Update Target Version" in the drop down at the bottom of the page and clicking OK. This will prompt you for the version to set. Once this is set, you will find some content under the Roadmap page.