Sorry, I found it. I thought I had done a thorough search in the InDesign Directories but I hadn't.
The Hunspell dictionary files for InDesign CS 5.5 can be found at C:\Program Files\Common Files\Adobe\Linguistics\5.5\Providers\Plugins2\AdobeHunspellPlugin
You will need to add any extra languages to Info.plist in that same directory as well.
Info.plist is generally a mac-only concept. Huh.
I was under the impression that you could add language dictionaries from Preferences the same way that you could add user dictionaries. But perhaps that is not true!
Yeah, I saw someone mention something like that in another forum, but I tried it and InDesign couldn't see the .dic and .aff files that are associated with a Hunspell dictionary, I think because the dialogue is only for custom user dictionaries.
I actually didn't try not editing the info.plist, I just assumed I needed to modify it since I had to do so on the mac.