Maven can be downloaded at: http://maven.apache.org/
At Ubuntu it can be installed with:
> apt-get install maven2
Build the project
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> cd webgoat
> mvn install
delete artifacts from previous build:
> mvn clean
Build the Eclipse project files
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> mvn eclipse:eclipse
Afterward the project can be imported within Eclipse:
File -> Import -> General -> Existing Projects into Workspace
Don't forget to declare a classpath variable named M2_REPO, pointing to ~/.m2/repository, otherwise many links to existing jars will be broken.
You can declare new variables in Eclipse in Windows -> Preferences... and selecting Java -> Build Path -> Classpath Variables