Citizens of most countries do not need a VISA to visit Mexico.
To make sure, please take a look at the list of countries whose citizens "do not" need a VISA to travel to Mexico: http://www.sectur.gob.mx/guia-de-viaje/visa/ , as well as the list of countries whose citizens "do" need a VISA to travel to Mexico: https://www.inm.gob.mx/gobmx/word/index.php/paises-requieren-visa-para-mexico/.
You can also use the tool provided by the The International Air Transport Association (IATA) to figure out.
The organizers recommend that you enter Mexico as a tourist. In case you need a formal letter to satisfy attendance requirements from your institution or funding agency, please contact the local organizers ( Prof. Jesús Martínez.)