Numbers won't open my numbers file, telling me the index.xml file is missing. Very frustrating.

You have Numbers '09 and are attempting to open a newer Numbers (v3/v4) document that is not backwards compatible with it. The document formats are different, and that is the cause of the dialog.

If you have Numbers v3/v4 installed, you should quit Numbers '09, and then right-click on the spreadsheet. From the secondary menu choose Open With -> Numbers (default) v3.

If you created the Numbers document, or even if you downloaded it from the Internet, it will have your user name, and read/write permissions on it. You can check this, by clicking once on the Numbers document icon, and pressing command+I (cap i). The permissions information will be at the bottom of the Get Info panel that you just launched.

If your name and permissions do not exist in the permissions area, you click the gold lock symbol, enter your administrator password, and then click the + symbol to add yourself not as administrator, but your normal user name. Assumption: the Numbers document is on your boot drive.

In the Finder, select the file, then select the menu item "File > Get Info" Is the file locked? if so, this will make the file read only. Uncheck the box, then open the file in Numbers 3.x