Hello Sara and Everyone else,
I have a solution that will resolve the issue of converting you Mac’s video output to SDI. The frame rate of the Mac’s video signal is the problem. If you use a DECIMATOR MD-HX Miniature HDMI/SDI Cross Converter with Scaling & Frame Rate Conversion, you can effectively re-clock the frame rate of the MAc’s video output to the proper timing for your SDI input. I was dealing with the same issue and this is the only device that I have found that can resolve this video issue reliably. The MD-HX can be purchase from Amazon for $295.00 and it is worth every penny!