I never had this problem until today, when I decided to try turning off the "check spelling as you type" feture. Now, as others have described, "add" is greyed out when I preform the spell check / also when I re-enable "check spelling as you type."
I made sure that Custom dictionary was checked, and that my language was US English. I can even open up my custom dictionary and see all the words I've added over the last year. But for some reason, when I run the spell check now, I get a message that reads: "The custom dictionary 'Custom Dictinoary' is not available." What gives?