Did you use a preview version of the iWork apps? Did you remove the trial version of Pages from your Mac before buying it from the Mac App Store?
If you did have a trial version and did not remove it then you need to start over. Uninsttall Pages from your Mac. You might download ind use the free AppCleaner to be sure that you get all of the pices. Restart your Mac and then download Pages again from the MAS.