The conference will take place in the Gromada Centrum Hotel shown in the map below. The best way to get from the airport (and virtually from any place in Warsaw) to the hotel is by TAXI. The ride from the airport costs ca. 40 PLN, an equivalent of ca. 10 EUR. Please take a taxi of one of three official airport taxi corporations: see a list here. Their cabs are standing just outside the arrival hall. There are also their stands inside the arrival hall where you can arrange for a cab accepting credit card payments etc. DO AVOID other taxi drivers who may try to offer you their services already inside the arrival hall - they will charge you essentially higher.
A detailed information on the public transport system in Warsaw may be found here http://www.ztm.waw.pl/index.php.