You can download the latest release of CommandPost as an DMG package here.
Once downloaded, simply open the package by double-clicking it.
You will be presented with a MIT license agreement like the below - click Agree if you accept.
The DMG package will open, looking like this:
Double click the CommandPost icon. It will now ask you if you wish to move CommandPost to your Applications folder:
Click Move To Applications. CommandPost will now launch the Welcome Screen.