The citizens of many countries (especially European Community) are exempted for obligatory entry permission (visa). The list of those countries are at:

www.msz.gov.pl/en/travel_to_poland/entering_poland/visa_free/visa_free_countries

The citizens of other countries must apply for visa at Polish diplomatics posts. More details you can find at the website:

www.msz.gov.pl/en/travel_to_poland/entering_poland/

The acceptance letter for presentation should be sufficient for obtaining a visa. Please check the information in the Polish Consulate and if preparing of special invitation would be necessary please inform Organizers (and send the copy of passport – data sheet).