Funilly enough I spent some time yesterday evening trying to track the file down. I added a custom word to my dictionary and searched all files in the Program Files and Application Data folders and got no results. Where ever the thing is, it is well hidden.
On Windows 7: C:\Users\<username>\AppData\LocalLow\Adobe\Linguistics\Dictionaries\R oboHelp Personal Dictionary
RoboHelp still uses the added.clam file. Whenever I add a word to the dictionary, the file is updated. I can even find words I added in the file. (Not all though, the file is rather garbled.)
Thanks, William, for posting the path. I never would have thought looked there! What I have (in RH10) is a subfolder called en_US, and then in there are files called added.txt and exceptions.txt. The added.txt file contains each of the words I've added to the dictionary, with no spaces between them. (There are numerous folders for other languages, too, each with a 0 KB added.txt file and exceptions.txt file.)
<PS> This also explains why when I added "e-mail" for a product whose product manager prefers it that way, it's marked wrong in the product in which I use "email" (because that product manager prefers it that way). I'd prefer to have project-specific dictionaries for cases like that.