To add this widget to your Enketo Express installation see this guidance.
Works on all regular (non-minimal) "select multiple" questions with the "literacy" appearance. It cannot be used in conjunction with other appearances. It works with all themes, but the Grid Theme is likely most suitable.
Note that this widget is a custom hack for which we cannot device a proper sane XForm syntax. It is therefore not suitable for inclusion in the common Enketo tools and the ODK XForms specification.