It's also possible that you have the latest version of Numbers on your Mac, in your main Applications folder, but you are inadvertently trying to open the document with the older version of Numbers, located in a subfolder named iWork '09. Any icons you have on your Dock may still be pointing to the old version.
Wayne Contello has written a User Tip for working with the old and new versions of iWork.
The trick is to drag icons for both versions to your Dock. Right click or control click on each and Options > Keep in Dock. That puts you in charge of which version you launch.
Here is a screen shot of both Numbers icons on my Dock.
Numbers 2.3 (Numbers '09) on the left with a three column graph.
Numbers 3.2 on the right with a four column graph.
Both versions will run happily together. Just be sure which version is opening your document.
Instead of double clicking the document, run whichever version of Numbers/Pages/Keynote suits and use File > Open.