I'm sorry to hear you're having this problem with your phone. As I stated previously it's been my experience that Samsung phones have many options that can be turned off or on which will affect the phone's behavior. If what I did doesn't work for you then I think you will have one of two options: 1) Continue to hunt through the many screens and turn options on and off until you find what's been turned on or off that is preventing you from adding contacts; or 2) Return the phone for a replacement. I wouldn't bother with a Samsung store, they won't know how to fix it either.
My experience has been, nobody on this forum knows what to do in order to correct this. As you've already experienced many people will continue to insist that it works just fine, there really IS a plus sign, you just can't or won't see it - presumably due to some sort of selective blindness. In fact the blindness is theirs: "It works for me!" does not equate with "It works for everyone".
Good luck!
