How to use Kurzweil synth patch with a controller in MainStage?

I'm confused as to how the Kurz would be connected to the "sound card" with a 5 pin MIDI DIN but must assume that whatever you've done results in both the Kurz and the 88 key controller being able to play AUs in MS.

To do what you wish to achieve, you'd need to route MIDI from MS to the Kurz. This could be done using a USB to 5 pin MIDI DIN interface with USB plugged into the MAC and the 5 pin DIN MIDI output port routing to the Kurz. Note that the Kurz MIDI input will be the name of the USB to MIDI convertor, NOT the Kurz.

You should then be able to route the 88 key to the Kurz using an External MIDI strip.

Best of luck!

I'm still not really understanding the Kurz 5 pin connections to sound card - what is the sound card and how is it connected to the Mac? If you have connectivity as implied by your reply, using an external MIDI patch you should be able to set its MIDI input to the 88 key and its output to the Kurz (but suspect you cannot select the Kurz as it is not really connected to the Mac but can you see the sound card as an output and use that as perhaps it will route to the Kurz as you say it is connected to it?). Sorry, I'm having to guess a bit at your set up. Best wishes.