The citizens of many countries (especially European Community) are exempted for obligatory entry permission (visa). The list of those countries are at:
The citizens of other countries must apply for visa at Polish diplomatics posts. More details you can find at the website:
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).