I know this is a worn out question all over the Web, but nobody on any string has a good answer.
I get the plug-in missing warning as many others have stated. I am the sole user of InDesign at my company, I never upgraded from CS3. All I have done is get a new computer and re-loaded CS3 from CD.
Files won't open, yes I am upset. Typical Adobe forcing un-needeed new upgrades on people in my opinion.
All I can tell you is CS3 files don't throw those missing plugin errors in CS3, so you've misremembered what version you were using.
You can determine the version the file actually is by running this script: Adobe Forums: [Ann] Identify Your InDesign File
Let the record state my foot is in my mouth. I indeed had CS5.