In order to handle a variety of low level mappings, the MusicBrainz Wizard requires that some of the Classical metadata items map to custom fields. If the custom fields are not defined, the metadata cannot be imported. A Custom Panel template is available which displays all of the classical fields which can be imported from MusicBrainz. The panel contains custom field definitions as well. The panel is described in more detail and may be downloaded from here.
This menu item will attempt to directly import the required fields. If differences are detected between current mappings and those being imported, you can safely choose to keep the current mappings.