I'm puzzled - OSX highlights words that it thinks are spelled wrong (or aren't in its dictionary), and when it offers me alternatives (in the contextual menu) to the word I've typed, it also lets me add the word to the dictionary. And sometimes, it offers no alternatives, but still lets me add the word to the dictionary. But for some reason, sometimes when it has no alternatives, it won't even let me add the word.

I'm writing this novel with some names in it from Greek mythology, and it's really annoying that OSX won't let me add them to the dictionary. Does anyone know of an app or service to let me do this manually?

Is it the OS or the application? I didn't know you could add words to the system dictionary, at any rate.

Well, no, not the system dictionary itself, to the user dictionary for the language you're using (en_GB in my case).

It turns out to be an app-specific problem (Scrivener) which requires that the app itself be set up specifically to use the en_GB dictionary for spellchecking.

Meanwhile, I've discovered Dictionary Editor which is an app that allows you to add, edit, and delete items from the user dictionary. There's a bug as of OS 10.6.4 which the dev is trying to sort out, where you have to kill the AppleSpell.service process before saving your dictionary after editing, and then log out/in to reload it - but he says he's on the case, and this doesn't happen in 10.6.3 and earlier.