Citizens of certain countries require a visa to enter México. In the following link you will find a list of citizenships which require visas.
Paises que requieren visa para México
Exceptions apply if you have a visa or if you are a permanent resident of the United States, Canada, or Europe. If you are a citizen of a country requiring a visa please contact the closest Mexican consulate at:
Consulados de México en el exterior
If you require an invitation letter in order to obtain the visa please email us at email@example.com specifying your current affiliation and title of the paper that you will be presenting.