Hello and welcome.
Usually that is a sign that you are trying to open a file that was created in a newer version of Numbers (version 3) with an older version of Numbers (version 2 - from iWork '09.)
-Do you have two versions of Numbers installed, i.e. iWork '09 and the latest version from the App Store? Both will be in the Applications folder...one called Numbers and one in a folder called iWork.
C