If you want to add a word to the spell checker's user dictionary, then you can use the integrated editor to perform a spell check using Tools->Spell Check (you cannot add words using the validator engine). When performing a spell check in the integrated editor, you will be given the option to add misspelled words to the user dictionary.
If Live Spelling is enabled, then misspelled words are underlined with red squiggly lines. These words can be added to the custom dictionary by right-clicking on the word and selecting the Add option from the context menu that appears.
You can also add words by going to the Validator Engine Options and the Validator Engine->Spell Checking Page. Press the 'More Spelling Options' button and then use the Custom Dictionary Tab in the Spelling Option Dialog. Select the dictionary that you want to add words to then add the words using the Adds tab. You can also remove words in case you've added a word by mistake or other reason.
Additional dictionaries for other languages are available and can be downloaded from our website.
For more information about spell checking, please see the Spell Checker topic.