I was going to suggest you check the settings Wiganken is showing in the previous reply.

I'm on a newer version of Word, but to get there in any of the ribbon-based Word versions, I think it's the same.

File/Options, then find Proofing from the left hand menu, and look for the "Custom Dictionaries..." button. If you don't have a Custom.dic file established, maybe it wouldn't let you write to it.

- Ann