Public transport in the Netherlands works and it is relatively cheap. To get to the 7S tournament from the centre of Amsterdam catch the bus 21 from just in front of Central Station. In the mornings they leave Central Station every 15 minutes and it takes about 30 minutes to get to the tournament. Trams cover most of the city and are quick and easy to use and almost all end at Central Station. A €2.90 ticket will get you to most places in the centre.
Please note that cash payments in public transportation is no longer possible! For people visiting for the weekend it makes sense to get a 3 day ticket which allows you to travel anywhere in the city at any time on busses and trams. It costs €17.00 and you can get them from the GVB office at Central Station.
For more info visit: http://en.gvb.nl/gvb-dag-meerdagenkaart