While the Validation Toolkit pdf (which incidentally comes up with an error message in my browser) does state that it needs CS4, perhaps it would be a good idea to also state on your info pages when a product is no-longer compatible with the current version. It would have saved me and perhaps a lot of others a lot of wasted time.
Do you have a list of those extensions that now are compatible with CS5.5, particularly as CS6 is now available?