Hidden Fields

In a project, I've created two new drop down fields. When I attempt to add the new fields to a different project I can't see or search for the new fields because I'm presented with a 'Hidden fields'. (See above)
I didn't originally create the project but I am the Jira Administrator and owner of the project.
Any ideas how to unhide this so I can select the fields I want to add to an issue type.
I'm conscious of messing anything up in the settings so does anyone have any straight forward suggestions.
Thanks
Jodie