Unable to open files, dialog mssg, 'The required index.xml file is missing' any suggestions

There are two general categories of Pages documents: 1) those created by Pages '08 and '09, and 2) those created by Pages v5 and later starting in Oct 2013. The Pages versions from (1), cannot open the newer document formats from (2), and the missing index.xml dialog is the proof.

Quit Pages from the application menu. If you have Pages v5 thru v10 (depending on your current operating system version), then right/control-click on the same document and choose Open With > Pages (default) (version > 5). If all you have is Pages '09, then you can open that newer document format with Pages for iCloud.