Hi, Megs. I haven't tried it myself yet, but I think you enter these codes where you would usually put your name for ranking after you win/lose a game. :smt024
I'm not sure why it's not letting you, but generally, to enter your name, you have to tie or surpass someone else's ranking. If you don't, then it won't give you the chance to enter your name.
Or, if you wish, reset all the scores and get a few wins (not good if you want to keep scores, though... :smt012 )