The hook will be called with a pointer to the entry to be displayed in A1 and a pointer to a string array containing as many entries as your list may have columns in A2.
You must fill this array with the strings that you want to display.
Note: You can of course use MUI's text engine facilities here to create e.g. right aligned or centered columns.
Without a display hook, MUI expects a simple one columned string list.
See MUIA_List_Format for details about column handling.