I think the scram doors are exclusive to the DG-XR1. You can see the button in the screenshot. If you haven't done so, I suggest you download this add-on for practicing.
Scram engines only work in a certain range of dynamic pressure. They won't do anything if your airspeed is zero before take-off. You have to activate them later, at the right speed and also stop them later at the right speed, while flying a very precise ascend. If you do it efficiently, you'll run out of scram fuel just before they overheat. Go read the XR Flight Operations Manual included in the doc folder of the add-on.