For those who dont know what is MAG ill give you a bit of insight.
The game will utilize a new server architecture to support online battles with up to 256 players, with users divided into 8-player squads, with 4 squads forming a platoon, and 4 platoons forming a company.Each squad will be led by a player who has advanced through the game's ranking system.Character statistics and development will also increase with frequent gameplay. The players most advanced in rank will be able to either direct the battle or participate directly in combat.
Steps to get the Beta:
1. Go to the PSN store
2. Select “View All By Title -> M Titles -> MAG
3. Download 'MAG BETA'
4. Launch the MAG Beta program from your XMB to download a special patch that will restart MAG once the update has been received
5.After the reboot, the Beta will reconnect and download the actual game data so that you can play
Enjoy the beta because it concludes on January 10th 2009.
Source: QJ.net