You opened your emWave program (Pro or emWave2) on your Mac and the Start arrow is grayed out.
You may even be able to run a session using one of the Games or Visualizers, switch back to the main Session screen during that session and everything appears as normal.
But as soon as that session is over, the Start arrow is grayed out again.
This is a rare, issue that happens only with some Macs and not at all on PC's.
If it happens, it's usually after an OSX upgrade.
This can be caused by a missing font in your Mac or one that isn't enabled.
The font in question is either Arial or Arial Rounded Bold MT.
You can follow the instructions here to enable the font.