Download the software here:
Click on the dmg file (downloaded file)
You may get an error saying it can't be opened.
The trick is to hold the Control button down on the keyboard when clicking the .pkg installer file.
And then clicking Open from the dialog. It may also require that you enter a password at that point.
Control-click the app > Open. Click Open on the dialog that appears.
Follow through the Installation Wizard.
Open the software.
If you've already purchased the software, click activate after purchase and request an activation key or paste in the key you've been given.