Your D-Name (screenname)? Well, you have to enter a username. It's hard to explain, so I recommend you watch this video (by Candifloss):

02:59

If you follow the steps, your D-name should be submitted for moderation. Just wait awhile (according to Disney, 72 hours!) and they should approve your name soon if it is appropriate. If not, try again and wait for your name to come again!

Good luck! :D