Q: since installing Yosemite cannot open numbers files...keep getting message "xml file missing"

You're trying to open a document saved in the current version of an iWork application using an obsolete version. Most likely, there is a subfolder of the Applications folder named "iWork '09". If you no longer need the old iWork apps, please quit them if they're running, then drag that subfolder to the Trash and empty. You may be prompted for your administrator login password.

If you prefer to keep the obsolete iWork apps, you'll have to select the iWork version you want manually whenever you open an iWork document. Instead of double-clicking the document icon in the Finder, right-click or control-click, then make a selection from the Open With menu.

Don't open an old-format document in the newer version of the app if you still want to be able to open it in the old version. Make a copy and open that instead.

Trying to manage incompatible versions of the same apps may be more trouble than it's worth. Eventually the old apps will stop working in a future version of OS X, and then you'll have no choice but to migrate.

You will have to redownload Numbers (and the other iWork apps, if you use them) from the App Store.

If you already paid for the apps, you shouldn't have to pay again, as long as you sign in to the App Store with the same Apple ID. Otherwise, you can use iCloud Drive and the iWork web apps at no charge, if you wish.

Use iWork with iCloud Drive - Apple Support