[DiapDealer]

You have to copy all three dictionary files together. You can't just toss the hyphen file in with the two pre-existing dictionary files (that came with Sigil). After unzipping the oxt file... copy the en_US.aff, en_US.dic and hyph_en_US.dic files (or en_GB.aff, en_GB.dic and hyph_en_GB.dic) to Sigil's dictionary directory.

Make sure not to mix and match with the older files. I still can't get it to work with the curly apostrophe and a user-added dictionary word though. But if the word is in the dictionary proper... everything behaves as I would expect it to. My emdashes are happy.