While it's not an official Mattel publication, it really is very cool that they're doing this. It keeps the fandom engaged and it harkens back to the 1980s contest and the Neitlichian contest... one of the few good things I can say about his tenure at Mattel.
So here's the ruling localized to English for the Spanish impaired:
1- This is an international contest.
2- in order to participate, you must send an image of your character as a JPG file. You should add the character's name and a brief description of the character... These (name/description) are optional. Also, I, NEFTY, am assuming that the Brief Description should be shorter than a MOTUC Bio.
Very important, you must add your name and contact email address to asociacionmundomasters at gmail dot com so the staff at Mundo Masters can contact you in case you win.