The Mac client requires you to download and install the ADOBE AIR Runtimes from the ADOBE website. Many applications run on Adobe Air and Flash Player so you should install these free products. Download from the link below.
These installations will work even if you are using the browser based client to play. You do not need to download these separately for the browser based client.
If you are using the browser based client to play, you will need the following installed on your computer. For most users, these may already be present. Follow the links below if you do not have the runtimes installed.