yes you are rigth, you can create a content field by clicking the button "create" and type the name of the content field. This content field can be used later in the process within the object browser -> process/activity data --> XMl fields. There you find your content field with the values siteUrl, ListUrl, FolderUrl, Item (this is the identifier of the item) ...
hope that helps

Where is the "create" button? I know how to use content fields once they are created, but there does not seem to be an option in the Move Document wizard to create it in the first place.

you can use the copy document wizard. there you have the option to create a content field.
If you want to use the move document wizard may be you have to select the option "move document from content field". I am not sure but may be that the content field is automatically updated with the new data

Ah, that worked to use the "copy" and "delete" wizards instead of the "move" wizard. Thank you!

OK, now I have the content field, but when I use the item ID in a notification, it always returns "1" instead of the actual item ID. Is there anything else I can do?

for test case put the root element of the content field in your email. then all the fields will be displayed. I think there is a field called "item" which stores the id? i am not sure..