Foreign nationals who intend to attend a congress may enter Mexico with the migration status and category of Non-resident Tourist; it is NOT necessary to obtain the FM3 form, unless they intend to engage in any paid activity.
Mexican law does not require you to carry an ID document with you - but it is recommended to do so. It is always best for visitors to make two copies of their passport identification page. This will facilitate replacement if a passport is lost or stolen. Leave one copy at home with friends or relatives. Carry the other with you in a separate place from your passport. Leave your passport in your hotel safe deposit.
Upon request, a letter of invitation will be sent to all submitters of scientific material, invited speakers, and invited participants from countries in which a visa is needed. For other participants, we kindly request you to send us official documentation of your professional affiliation with mathematics education. After receiving your documentation, an official invitation letter will be sent to you as well.
The government of Mexico requires that all U.S. and Canadian citizens present proof of citizenship and photo identification for entry into Mexico. Citizenship documents such as a certified copy (not a simple photocopy) of a birth certificate, a Naturalization Certificate, a Consular Report of Birth Abroad, or a Certificate of Citizenship are acceptable. However, it is recommended that travelers carry a valid passport to avoid delays or misunderstandings. Driving permits, voter registration cards, affidavits and similar documents are not sufficient to prove citizenship.
U.S. and Canadian citizens traveling as tourists beyond the border zone or entering Mexico by air must pay a fee to obtain a tourist card, also known as an FM-T, available from Mexican consulates, Mexican border crossing points, Mexican tourism offices, airports within the border zone and most airlines serving Mexico. The fee for the tourist card is generally included in the price of a plane ticket for travelers arriving by air.
As of May 2007, the Foreign Relations Office provides the following list of countries whose citizens are exempt of requesting a visa to enter Mexico. If your country of origin is not included in this list, please call your local Mexican Consulate for information on visa requirements.(list source)
For further, authoritative information, please check the list of embassies and consulates. Bear in mind that the visa granting process can take between one and three months; you are advised to start the process as early as possible.