I use GreenHopper's classic boards, can I use Rekall?

Sure you can! You just need to create a rapid board that use your JIRA's project and modify its filter to match the fix-version used for your product backlog. Rekall will then show you all the unestimated issues in your product backlog and compare them against the executed issues in your project.

I don't see any license tab in the admin section

When using later versions of the Universal Plugin Manager, JIRA may hide the Rekall License tab. If that is the case, simply perform all licensing operations directly from the UPM.

Rekall fails assigning story points but the JIRA story points field is properly set

It may happen that Rekall fails to assign story points and mention to validate the story points field in the admin section. However, the admin section shows that the proper field is assigned. If that is the case, select another value, re-select the proper value, save the changes and then try again. It is possible that the value was not properly detected and saved during the initial installation of Rekall.

Is the GreenHopper plugin required?

Yes, Rekall requires that the GreenHopper plugin be installed in JIRA in order to work propely. Since Rekall is getting some of its data directly from the services provided by GreenHopper, Rekall will display an error message if GreenHopper is not installed.

Rekall fails to load after install

Rekall relies on GreenHopper to work properly. In some instances where Rekall was installed prior to GreenHopper, it is possible that Rekall fails to load the Triangulation Board. If that's the case, try to uninstall Rekall and re-install it after GreenHopper has been installed. Disabling and re-enabling Rekall is not sufficient. You have to uninstall and re-install Rekall.