[eschwartz]

Drag-and-drop the ebook file onto the Book Details pane on the right. It will add the file as a new fomat for that book record.

You can also use the Add book format option in the Edit Metadata window.

Copying the file into the library folder via the file browser won't work, because it doesn't tell calibre that the file is there. You need to add the book format from within calibre.