I downloaded the free trial edition of World of Warcraft and when I start it up, a message box pops up and say?

I downloaded the free trial edition of World of Warcraft and when I start it up, a message box pops up and says " Launcher requirers write permission to the C:\program files\world of warcraft trial\ directory to succesfully patch the game. Please enable write access to the directory using an administrator account." How do I fix this?