You want a Private or a Commercial?
If Private you don't need nothing but ask for a convalidation and it will be given to you with no problem at all.
If you want a commercial...
It's a Big Brother, rules change every month and everyone has a different case.
I recommend you to approach "Licencias" at MEX
and go with your papers and show them your situation and what do you have to do exactly.
Basically you'll need to do the theorical CIAAC exam and a practical test + 10 hours of flight school in Mexico.
CIAAC's phone for you to ask for the next exam and papers you have to handle there is:
57239300 ext. 18654
Then go on to DGAIR of SEP to live the paper and multiple hassle of copies of papers and papers and payments.
DGAIR is on the 6th floor in a building near to Televisa Chapultepec at MEX
After that, get an appointment at "Profesiones" and finally go back to Licencias where you started to get your provisional flying license until you have your Cedula and Certificate printed.
This one is somewhere in Insurgentes Av. I haven't got to this step yet until January 13th 2012, that's the day they have free space.
It's insane and a total hassle.