How to use at the same time the audio sounds of a keyboard & the midi sounds with mainstage

Yes. To have a silent Mainstage patch, simply do not create any instrument channel strips. To have the SV-1 silent on a patch you should be able to set and save the volume for that patch to 0. As an alternative you could turn local off on the SV-1( this function will reset to local on after the SV-1 has been turned off). With local off you can determine with each Mainstage patch whether or not you want to transmit MIDI to the external instrument(SV-1) channel strip. If you have local on, be careful not to create a MIDI feedback/loop with the SV-1. This is a lot of info to digest at once, so take it slowly, step by step. Snapshots below are somewhat in order.

Click on the cog wheel in the upper right corner of the channel strips section to get to this menu. Select add external instrument channel strip.

The external channel instrument strip will look like this.

Click on the MIDI output box to show the display below and choose your external keyboard destination(SV-1).

In the channel strip inspector you can send patch changes to the SV-1.

Here is where you select the MIDI input for routing MIDI to the SV-1. If want no SV-1 sound for this Mainstage patch, select none.

You'll need to play with these parameters to get the desired results. Stepping through the MS patches with a footswitch can be done, too, but that can wait for now. Post another question when you master these steps.

but my keyboard is not listed on my MIDI Output list.. What should i do now??