Could it be that you're talking about raw files here?
With older versions of TP, you could only edit the metadata of JPG/TIF/PNG files, but v10 supposedly allows for the manipulation of metadata in raw files, too. However, this requires that you first activate an option "Allow creation and update of metadata in raw Digicam files" in the Options | Preferences | Metadata dialog. So, maybe that's what you have to do in order to get this to work.
The Help entry for this setting says: "ThumbsPlus can update metadata in most raw files, but after updating, some cameras will refuse to load the info. You can enable this option as long as you're not editing the metadata on the camera directly (through a mounted device), and as long as you don't plan to upload your modified images to the camera."
(btw, personally I would never write to my original raw files, but I guess working on copies would be okay).
Image | Batch Edit Metadata in the main menu is another way to edit the metadata of one or more files (if it still doesn't work through the metadata panel).