That means the file you recieved was saved in CS5.5 or CS6
You'll need to ask the provider to export to .idml for you to open it.
Knowing that there isa format that is importable is useful - thanks!
So do I take it that there ARE no 'updates' for plug-ins?
Correct. The latest update to CS5.5 finally gives the user the proper message that the file was created in a newer version of InDesign.