The camera is giving you a correct liveview image based on the ambient light in the studio, but cannot possibly show you what will happen when your flashes fire. So if you have set exposure to something like 1/320, f/8, ISO 100... the image will appear super dark in the EVF because you are dramatically underexposing the ambient light of the studio. The solution is to therefore turn OFF Exposure Simulation and the camera will give you a normalised correct exposure in the EVF/rear LCD. It will make the exposure appear to be correct, regardless of camera settings, but is not showing you what you are actually going to get. It will allow you to use a bright viewfinder and see what you are focusing on.
Edit: one thing I forgot to mention is that tapping the shutter button momentarily disengages Exposure Simulation so that you get a bright clear preview, but it reverts back to Exposure Simulation about 2 secs later. So you can leave Exposure Simulation on, if you wish, and just keep tapping the shutter button each time the EVF goes dark. It is less of a hassle to just have Exposure Simulation turned off doing studio work. Make sure you remember to turn it back on afterwards, or you might be wondering why changing settings in Manual mode is not changing the image in the viewfinder.
