Need newer version of Numbers to open file

If you have ungraded to Mavericks and also installed the latest iWork Applications (Numbers, Pages and Keynote) you may see this dialog appear:

If you see this, it is because you are using the previous version of the application while attempting to open a new-format file. Both the previous and new versions of the iWork applications are installed on your computer.

The New versions are located in the folder "/Applications" while

the previous versions are located in the folder "/Applications/iWork '09"

They have distinctly different icons:

It is a good idea to know exactly which application you are launching before you try to open a document. One thing you can do is keep the version want to use in the Dock by dragging the application from the Applications folder to the Dock.