Note: Update database does not update extra fields such as Artist, Rating, Legality, etc. See below to update them.
First of all you need csv file with cards, usually missing cards are promo cards, which actually exists in another set but were reprinted in some specific set, in this case only fields you need are: NAME|SET|ARTIST|COLLNUM|IMAGE_URL.
In Card Navigator view select Import button, select Extend database options in the bottom and select csv file to import.
When you press Finish it will tell you that set does not exist and you should create a new set and enter abbreviation for this set.
Then your cards should be in the database, and you can add them to your collection or decks (as separate step).