One thing you can try is registering your cd keys onto battle.net. Then you can go ahead and download the mac installer straight from the site so you know you have the latest version from Blizzard's side.
I know of two things you can try: first, you need to have an administrator account to perform the install, otherwise you lack the necessary permissions.
Second, you can download the current version of the game from Blizzard, by registering at www.battle.net. More info here.