[cart] callback customfield

have been searching the forum but did not find any help.

i would like to add the contents of the customfields of my images to be displayed on the viewcart.tpl page.

as i gather, i can call these values by using:

{g->callback type="customfield.LoadCustomFields" itemId=$item.id|default:$theme.item.id}

in my viewcart.tpl file. now i cannot see any of these variables showing up in the debugger. what am i doing wrong?