The Pokémon Trading Card Game Online has completely changed its front-end client for players and has ditched the Flash version. Players can currently download the Windows beta version from the Pokémon TCG Online's site. A Macintosh version will be available at a later time.

Press Release

The Pokémon Trading Card Game Online team has been hard at work rebuilding the game with new technology and a fresh look. The first game mode updated is the single-player Trainer Challenge, and the upgraded multiplayer modes are planned to enter beta in a few months.

To focus completely on the new features, further work on the current version of the Pokémon TCG Online will be halted. Plus, the Multiplayer—Expert and Multiplayer—Novice game modes will be disabled until the new version launches. You’ll still be able to access your card Collection, the Deck Builder, the Trading area, and the Player vs. AI game mode throughout the transition.

Note that during the transition, you’ll be able to claim booster credits from code cards found in booster packs of the Black & White—Dark Explorers expansion beginning on May 9th. You can redeem these credits immediately for any expansion from HeartGold & SoulSilver to Black & White—Next Destinies, and you’ll be able to redeem booster credits for Black & White—Dark Explorers cards later this summer, when the new multiplayer modes enter beta. Similarly, theme deck codes will unlock Black & White—Dark Explorers theme decks in the Trainer Challenge right away, but they will not show up in your Collection until the multiplayer open beta begins.

Mac Client

Due to the new version of the Trainer Challenge requiring a downloadable client, we opted to complete and test the Windows version of the game prior to porting and polishing a Mac version. We understand that including the Mac system requirements in the community update earlier this month was premature. We intended to indicate that the Mac client was in the works, but it appears that this message gave the community the impression that it would be launching at the same time as the Windows client. We apologize for this confusion.

The Mac client is scheduled to release later this year. We apologize for any disappointment this decision causes our Mac users and would like to assure you that this decision was not made lightly. We will provide schedule and development updates on the Mac version as the launch window comes more into focus.

Source: http://www.pokemon.com/us/pokemon-news/tcg_tcgo_update-2012-05-18/