You need the Google App Engine java sdk. You can get it here.
After downloading and extracting it, make sure you add the bin directory to your PATH. You will also need to install leiningen before you begin. Once you have completed the install simply run
lein deps
cp path/to/appengine/lib/appengine-tools-api.jar war/WEB-INF/lib
cp path/to/appengine/lib/user/appengine-api-1.0-sdk-1.3.3.jar war/WEB-INF/lib
lein compile
To start the app run:
dev_appserver.sh war
and visit http://localhost:8080