It is possible to buy single ride tickets, but a better option to pay for tram, metro and train is to buy an OV-chipkaart and load some money on it. Slide the card against a card reader both at entering and exiting, and the required fee is deducted from the balance on the card. The card can be bought for 7.5 euros at railway stations and many other locations, including Schiphol airport. Money can be loaded on it at these locations as well. The cost of a ride depends on its length, with 1.5 euro being typical. An alternative is to buy a five day card for unlimited travel. Note: card readers for the train and metro are typically located at the entrance of a station or on the platform, for trams they can be found inside the tram. At the De Boelelaan/VU stop use the correct reader for either tram 5 or metro 51. Tram 5 departs from the lower platform, reader inside the tram. Line 51 departs from the higher platform, reader on the platform.
See http://en.gvb.nl/pages/home.aspx for more details.
Unfortunately, the Amsterdam transportation company is doing maintenance on the
metro system during the conference. The metro is still operational, but it might be necessary to change more often than usual, causing some delays. In particular, metro 51 can not be taken directly from Amstel Station (for those staying at CASA 400). Instead, first take metro 54 to “Spaklerweg” (1 stop) and change to the 51 there.