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I was getting a dialog box with these messages when trying to play DRM'ed music or trying to login to the iTunes store from a MacBook. (This is a message typically seen on Windows.)
I spent around 6 hrs on the phone with Apple support (they were very nice). We tried everything from removing and redownloading iTunes, to deleting preferences, to creating new Mac OS X accounts (which worked, but didn't solve the problem of accessing the iTunes store from my primary account).
Bottom line, the solution was to run Keychain Access --> Preferences --> Reset my default keychain.
After doing this, I was able to login to the iTunes store, and play DRM'ed music. I'm posting this in hopes that it will solve someone else's iTunes access problems!