"The required index.xml is missing" in Pages and "You need a newer version of Pages to open this document"
You need to go to the Mac App Store and install the latest version of Pages (5.x)

As of now, the latest version of Pages is 5.5.1, and it's only available from the App Store.


Mac App Store
Apple website




I've seen this when using Pages from iWork '09, which is 4.3. The file format changed and iWork '09 cannot directly open the new files.

You can export iWork '09 compatible files from Pages 5.x though.