This is a video driver issue. With Macs that means an OS X update, since they come with the OS. Any chance you can go up to, say, 10.9.5 ?
Also, if your video card is not up to spec even that may not help. If you're not sure what you have and how much video memory it has, look in Steam-Help-System info, the Video card area - we want the first line (Driver) and the last line (VRAM).