New WD Passport doesn't work straight out of the box on my Mac

I have just purchased the WD Passport but it doesn’t work straight out of the box on my Mac, I have searched the web for a solution installed the WD Discovery for Mac dmg. I have read that I need to format so went to Disk Utilities selected Mac OS Extended (Journaled) then this message came up… Couldn’t unmount disk…operation failed…any help would be so appreciated please.