They are packaged as two m2eclipse projects, and it is recommended (although not necessary) to import them into Eclipse to use them.
groovy-eclipse-compilerproject contains the compiler integration. It is a single Java class that calls into the Groovy-enhanced JDT compiler. This maven plugin uses plexus to hook into maven's compiler plugin.
groovy-eclipse-maven-testsproject is a very simple maven project that has a few Groovy and Java classes that interact with each other. If you want to create your own project using the Groovy-Eclipse maven integration, I would recommend starting with this sample project.
Please let me know if you have any problems.