Apologies! I thought I had replied… I know I started to write one but I can not remember actually submitting it.
re: when I go to enter a new directory entry using the Form from the frontend as an user, I cannot find an field for Custom Field input.
Sorry, but the Custom Fields metabox is available in the admin only. It is not supported by Form. The reason being that this is a field that you generally would not want users being able to added/edit because they could add/edit the values to be anything that they wished.
re: I cannot edit Custom field data in entries which already have a Custom field.
Sorry, I am not sure I understand. Could you give me more details?