MonoGame 3.5 is basically completely broken on retina Macs. All mouse coordinated are way, way off and you can’t click and drag. Oh yeah, and the window and scaling will all be half so you’ll have everything super tiny.
If you just want to give the demo a quick look move the window so it is in the upper left of your screen then move your mouse around it until the hover effect on the buttons shows. The cursor won’t actually be over the buttons but they will still work. You can then just click to activate.
For a proper fix what I usually do is drop back to MonoGame 3.4 which does actually work on Mac.