You have to select a device as MIDI input source always, but not an interface itself. This is the only way to tell OMS or FreeMIDI from which port (as some interfaces have multiple ports) MIDI can be expected. Therefore the device must be created in the Studio Setup of OMS or FreeMIDI. So create a device there, virtually "connect" it to the interface which is shown in the studio setup already (if properly installed and recognized by OMS or FreeMIDI). When connected make sure that the port settings are correct for the connection you have made.