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Once you have checked out the project you should see something like this:
Access to Needed Jar Files
You need to make sure the appropriate jar files get into your class path. These are kept in the src/webapp/WEB-INF/lib directory as well as in the top directory.
Question to Matt - why are these not in the proper places by default??
If you didn't get them in the right place, they are generally to be found in the lib/runtime directory.