Can't add words to dictionary

I’m using EndNote 6 on a PC platform. I’m trying to get the SpellCheck to work. I have selected language preference and a dictionary (Briish English - sscebr.tlx ). The “Add” button is active in SpellCheck. But when I click it, I get the following message “Can’t add word to user dictionary”.

What do I need to do in order to get this function working?

Thanks!