• Launch Steam and go to File > Settings
  
• On the 'Account' tab of the Settings window, under the 'Beta participation' section, click the button that says "Change..."
  
• Select the Dawn of War 2 Beta from the drop down list on the pop-up window.
  
• Click OK and you'll be prompted to restart Steam.
  
• When Steam starts back up, your copy of Dawn of War II will automatically update to include the beta content.
  
• Launch the game to enter the beta.
  

Included in this update:

• Significant balance update to the multiplayer game
  
• Battle over eight new multiplayer maps.
  
• Edit, create, and add new maps to multiplayer games with the map editor World Builder.
  
• Watch custom multiplayer games as a spectator with the new Observer and Referee options.
  
• Plus, improvements in performance, user interface, audio, and artificial intelligence
  

Read the full release notes here