Blizzard titles won't install on Macs with FileVault® or FileVault 2 enabled. To disable FileVault or FileVault 2, follow the steps below:

  1. Select System Preferences from the Apple menu.
  2. Click on Security & Privacy.
  3. Click on the FileVault tab.
  4. Check to see if FileVault is enabled. If it is, click the Locked icon, then enter an administrator name and password.
  5. Click Turn Off FileVault.

For more information, visit Apple's FileVault knowledge base article.