Not ever had to change spell check but from BD help the following may help.
The user dictionary is stored in "spell_check_dictionary.txt" file, in Documents and Settings\User Name\Application Data\Brilliant Database... folder. You can manually enter here all specific words.
Just did a search on my computer and your right - no txt file with "spell" found in BD folders or even a windows file search. Why the the help file mentions that location is a mystery - if BD support would reply they could answer in a flash.
Possible slow alternative is create a simple form enter or import words and then do a spell check - adding words as required by clicking add to list. Need to check if this is stored only in current database file or permantly in BD before spending too much time.
Sorry about this but assumed BD help would be accurrate with some thing so specific.
UPDATE. Seems dictionary is stored outside database file - remembers added words when new database created.