Open-Sankoré does not work with OS X Yosemite. If you run the program in OS X 10.10, Open-Sankoré closes immediately. The only workaround I have discovered is rolling back to OS X 10.9 (Mavericks). The Open-Sankoré website does not provide much insight on when this issue might be fixed. However, it looks like there may be an alternative…

OpenBoard

Using Google Translate, I can see that SEM-Logistics and Christian Oihenart have released a version of OpenBoard that works on OS X Yosemite (although there is apparently a small bug in the display of the dock). I have not installed or tested OpenBoard, and have no idea how well this might work, but it is now on my radar.

Update: 30th July, 2015

The old link to OpenBoard is dead. This new one seems to work.