In Modul8 2.5.x if you have connected your Midi device AFTER you launched Modul8 you will need to access 'Mapping > Rescan Midi devices' from application menu.
In Modul8 2.6 scanning of Midi devices is automatic. When you connect your Midi device Modul8 will detect it and you will be able to map Midi commands immediately using 'Mapping > Edit Midi Map.'