Although I've only used it once, it appears that my Heroes of Might and Magic V CD 3 doesn't work.

I noticed that the game is available on Steam, and I've been trying to activate the game on Steam, so I can install it on my new computer. However, it rejects my CD-key, whether I type it in upper or lower case, with or without dashes.

Is there some other trick to activating Heroes V or its expansions on Steam?

Thanks!

Edit: It appears that being available on Steam doesn't necessarily imply that the game can be registered on Steam. Maybe I'll think twice before getting a CD version of a game next time.