http://www.consulmexny.org/eng/z-bak-visas_fmvc.htm
FMVC for permanent residents of the United States and Canada
If you are a permanent resident of the United States or Canada, who wishes to travel to Mexico for business purposes and for less than 30 days are now allowed to do so with a migration form known as the FMVC form. The FMVC is the easiest manner to do business trips to Mexico.
You may obtain the FMVC form at the port of entry. You may also request one at this Consulate.
The requirements to obtain an FMVC Form are:
Valid Passport
U.S. Permanent Resident Alien Card or evidence of legal residence in Canada
Original Letter from the Company specifying the exact purpose of your trip to
Mexico and how your expenses will be covered.
If you are planning to go to Mexico on a business trip for more than 30 days, you need to apply for a nonimmigrant visa FM3, also known as business visa for multiple entries.