The field shows up on the issue pages (normal JIRA issue view) with non-editable value "inactive". But it does not appear in the Greenhopper "sort by" drop down list.
What am I doing wrong?
In fact there other existing fields (predefined or custom) which don't show as well up in the drop-down-list and I can't find a rule when a field will be displayed or not.
I think you missed this in the step by step instructions:
Register your GreenHopper ranking field for each project that is subject to use it from your project`s planning board configuration (in the General section)
So this is in the
Project > GreenHopper > Configuration > GENERAL >>>> Ranking Field
As soon as you add your field it should now be an option in the "Sort By" drop down of your context.