IMGW-PIB is located in the north-west part of Warsaw. If you arrive by train in the Warszawa Centralna rail station then get on tram no. 17 (Huta Warszawa direction) and get off at the Podleśna stop. It takes ca. 30 min. to get to the Institute and costs PLN 4.40.
Travel by taxi is more comfortable, it lasts ca. 15 min. and costs about PLN 40.
If you arrive by plane then go to taxi stop and take a taxi. Travel will cost ca. PLN 60.