Unknown file version number
You could be trying to open a newer KaleidaGraph file with an older version of the program. Version 4.5 can open any KaleidaGraph file, but earlier versions can only open files created by v4.5 if they were exported in an older file format using the Save As command.
It is also possible for defective files to display this error message. If the files are stored remotely (memory stick, network drive, etc.), you could try copying them to the local computer to see if that helps.