Air: Leiden is located close to Amsterdam Schiphol airport. From Schiphol there are trains to Leiden every 15 minutes. Take the trains in the direction of "Rotterdam" or "Den Haag" (but do not take the international trains to Brussels/Paris or the so-called "Fyra" as these do not stop in Leiden). You can buy train tickets at Schiphol. The easiest is to go to the ticket counters in the arrivals hall instead of using the machines. Note that public transport in the Netherlands now works with the so-called "OV chipkaart". It is the only way to get on the train, bus tickets can still be bought on the bus from the driver.
Train: Leiden can also be reached by international train. Depending on where you travel from you will have to change trains at Utrecht, Amsterdam, or Rotterdam to get to Leiden.
The Ibis hotel is located across from Leiden train station. The Holiday Inn can be reached by taxi or by bus (line 43 of the Veolia company, which has a stop in front of the hotel, see marker on the map). The Holiday Inn also runs a free shuttle service between the hotel and Leiden train station (working days only). For other hotels please see their respective web sites for directions.
The conference will take place at the Holiday Inn, which can be reached by taxi or by bus (from Leiden train station, line 43 of the Veolia company, which has a stop in front of the hotel, see marker on the map). You can also rent a bicycle. It will take about 20 minutes to get from Leiden centre to the Holiday Inn.