To add a moderator/editor, edit the category which you want them to be moderating and write the member's username into the "Moderators" box.

In your usergroup permissions, moderators are considered as regular members who own all links in their category and own the category they moderate as well. If you wish to allow moderators to edit category information, you can check the 'can edit own categories' box for the registered member usergroup.

You can refine moderator abilities more by setting a default moderator usergroup in your settings.