In order for you to be able to send MMS messages from Telcel, the person you are trying to send a message to, needs to have the service activated. Not all users have the service. In fact, users of Paid as you go or Amigo are not entitled to do so.
Anyway if they have the service activated you can send a message as follows:
If youre’ the MMS receiver, you have to go to http://mms.itelcel.com/servlets/mms to retrieve your MMS pictures or videos. You can also directly go to http://mmbox.itelcel.com/mmawap.
Quoted from Wikipedia:
Telcel is Mexico’s largest mobile phone carrier, owned by América Móvil. Founded in 1989 and based in Mexico City, Telcel is the leading provider of wireless communications services in Mexico. As of December 31, 2006, Telcel’s cellular network covered more than 63% of the geographical area of Mexico, including all major cities, and 90% of Mexico’s population. Telcel holds concessions to operate a wireless network in all nine geographic regions in Mexico using both the 850 megahertz and 1900 megahertz radio spectrums. As of December 31, 2007, Telcel had approximately over 57 million cellular subscribers and, according to Cofetel, as of July 2008, an approximately 77.2% share of the Mexican wireless market.