A family of Microsoft word processing software products for creating web, email, and print documents.
There are several incorrect things in your post.
First, you can indeed right-click a word with a wavy red underline and click Add to Dictionary. You don't have to go into the dictionary file to add a word.
The thing you need to do -- only once -- is go to File > Options > Proofing > Custom Dictionaries, click on CUSTOM.DIC (or any other custom dictionary that you care to create with the New button), and click the Change Default button. (The "default default" in Word's setup is RoamingCustom.dic, which isn't a great choice.) Make sure the chosen dictionary is set to "All Languages".
Once that's done, the Add to Dictionary menu item will add the selected word to the dictionary that's designated as the Default.
These things are covered at the end of this article: http://wordfaqs.ssbarnhill.com/EditCustomDic.htm