Go to download page and choose Mac OS:
Click 'Download' button to download .dmg file:
Wait until ClusterGX is downloaded:
Right-click on downloaded file and choose 'Open':
This action should add following icon to your desktop:
Click on this icon to open.
In the window that opened right-click on gex_x.x.x.pkg and choose 'Open'
Mac OS will warn you about untrusted developer - it's because beta is not signed yet. Ignore it and click 'Open':
Now just follow the installer instructions:
On the fifth step system will ask your password to install new software:
All done! Click 'Close' and application will be launched.