Your options are pretty limited at this point. If you bought a Suite package, you must upgrade the entire suite at once, but even if you have an individual license for ID, the odds of finding a CS5.5 upgrade are getting pretty slim since CS6 has been out for over a month. I'd check places like Amazon, though, where they might still have some old stock.
And no, you can't update the plugins. If you need to open the file in CS5, it will need to be exported to .idml from CS5.5 or CS6.
Hi Peter. Do you know anything about the free upgrade from CS5.5 to CS6? If I could get a CS5.5 upgrade quickly for the whole suite, would it save me money because I would get a free CS6 - or are the two the same price anyway?
You can't buy CS5.5 anymore and even if you could find it you wouldn't be eligible for a free upgrade...that offer has long since expired.
Best advice I can give now is to move to Creative Cloud at $29.99/month. That intro special ends on Aug 31.