The latest update to the Pokemon TCG Online included a desktop client for Mac users. This long awaited addition has been promised since Pokemon TCGO had became a desktop application in the Spring.
The Mac version of the desktop application will have the same features as the Windows application. The client will use the same servers as the Windows application. This will allow users of the previous web-based application access to their old collection that they had from the previous web-based version of the game. Both versions of the Pokemon TCGO will be able to interact with each other to conduct battles and trades.
Source: Pokemon Trading Card Game Online