I had a similar problem with the Yamaha MOX editor and I'm assuming it's the same issue for the Motif editor.
When the editor is fully working, it will receive MIDI from the DAW, route it to the Yamaha, have the Yamaha audio come back into it and output the audio. So it works very much like a virtual instrument. Unfortunately, this only fully works in Cubase.
There is a workaround though.
1) Create a Folder (Optional)
2) Insert the VST editor. You will only use this for configuring the Motif. You will not be sending any MIDI to it or receiving any audio through it.
3) Create a MIDI track for each MIDI channel you want to send to the Motif. Send each of them to the appropriate MIDI channel on the MOTIF hardware (not the VST plugin)
4) Create an audio track for each output you want from the Motif hardware (not the VST plugin). The MOX had 2 stereo outputs so I created 2 stereo audio tracks but that might be different with the MOTIF.
So basically you are using the VST editor plugin to edit and store the configuration of the MOTIF in your project, but not as an actual "Instrument".
Personally, I found it was a pain in the butt to use the MOX as an instrument my DAW and just decided to use VST software instruments instead when I'm using the computer. So much easier.
I now have a Korg Kronos and I use that as a standalone music production system. . . not with my DAW unless I want to record the output of the Kronos.