a) Use your zip program(I suggest WinRAR or 7-Zip) to extract explorer.war into ./explorer
b) Add gwt-user.jar and get-servlet.jar to ./explorer/WEB-INF/lib
c) Deploy the war as exploded on your app server(I suggest Glassfish)
a) Use your zip program to explore the explorer.war file
b) Drop the above libraries into WEB-INF/lib and your zip program should add them and recompress.
c) Deploy the explorer.war file.
The GXT download does not include or distribute the GWT jars so including them is not an option. For extjs.com, we put the GWT jars is the common library folder for Tomcat rather than adding them to each GWT application.