The above message instructs you to head to Privacy Settings to enable access, but it doesn't tell you exactly where to go. To expedite your time in settings, allow me to show you where you need to go.
Open Settings and choose Privacy:
Next, choose Photos...
... and you should see Facebook listed here if it has asked to access your photos (which it would have done when you first launched Facebook after installing iOS 6.):
If you answered Don't Allow when Facebook asked to access your photos, the slider here will be set to Off. Tap it to turn it on and you're good to go, uploading the photos of your kids or cats stored on your Camera Roll to Facebook at your every whim.
(Via iPad Insight)