On this site, there are a number of documents to help you get started with DWR:
Outside of this site there is a growing collection of tutorials from around the web.
There are various different ways to configure DWR depending on how you like to work:
Reverse Ajax in DWR 2.0 needs careful configuration to work around the various network problems that can arise. Reverse Ajax is Comet + Polling + Piggyback. See the reverse ajax documentation for more details.
DWR integrates with many external libraries like Struts, JSF, Webwork, Rife and so on. See the section on integration for more details.
It is common to need to pass extra data to callback methods and there is also a reference to how to handle errors too.