K-9 Mail not working with Openkeychain
Apparently on the "Accounts" screen, one has to long press the account.  Menu will pop up.  Click on "Account settings".  Another menu will pop up.  Click on the "Cryptography" item at the bottom.  The Cryptography menu will open.  Click on "OpenPGP Provider".  By default, "None" is selected.  Click on the "OpenKeychain" and you're set!

Hope this helps!