How large is your items.000 file? There is a limit on the number of entries the editor can handle, but I don't recall what that is off the top of my head (maybe 10,000?). It can read my 2.2MB items.000 file from a save at the start of the wastelands fine, containing 5922 entries.
Are you using v1.4 of
DAD's item editor (listed in the title bar of the program window)?
There is no way that I know of to create new items, as opposed to editing existing ones. In the item editor you can select and edit the listed name.