I could not add new words to a Microsoft Word for Mac custom dictionary

if you try to get the dictionary to accept spelling, it says the library custom dictionary is full. - that is an error caused by the document you are working in being set to a different language than the one you usually use. For example I received a document written in English (US) and I usually use English (UK). To fix this change the default language of the document (Tools/ Language/ select the language you want to work in, and click default at the bottom left corner of the pop up screen. It will set the document language to the correct one for you in the do, and you can add as many words as needed.