Sorry to take so long; I have a wife, small children, and a full-time job. These things tend to reduce my free time.
If you can still see the XDK submenu and are getting the error on shutdown, then you haven't fully removed my version of input_server.
To fully remove the input_server:
Open the zip folder again, double click on each folder. From the servers folder, remove input_server. (It should be the only file in there.) From the other folder, remove the packages file. (It should not be the only file in there.)
Since as far as I know, my input server patch has not been applied, doing this will make you unable to see the input method submenus, so you might want to wait.
To turn on the method:
Make sure the XDK method is selected. Its icon should be visible in the deskbar instead of the keyboard icon for the 'Roman' input method.
Make sure a map is selected. It should have a checkmark next to it in the submenu, and when you look at the top-level input method menu, the text should have changed from 'XDK' to the name of the menu.
Once that is in place, `, ', and ~ are the deadkeys.
You should use it with the main Haiku keyboard set to the Lithuanian keymap, because otherwise you won't have access to the base letters to use with the deadkeys.