The community uses IntelliJ instead of Eclipse, so I'd recommend you start by getting Briefcase to work first in IntelliJ so you can see how it's put together.
Briefcase builds using gradle, so that's going to be the key to getting it to work in Eclipse. http://www.vogella.com/tutorials/EclipseGradle/article.html seems to be a reasonable guide. I'd do that as the second step after you get it working in IntelliJ.
Oh and when you get a chance, please introduce yourself here. I'd also encourage you to add a real picture as your avatar because it helps build community!