forward Click on the link at the top of the page and download the RBZ file to your computer
forward Open SketchUp. Click Window (Windows) or SketchUp (Mac) > Preferences > Extensions > Install Extension…
forward Browse to the folder that has the RBZ file, select it and click "Open." You will see a warning message that asks if you're *sure* you want to install the plugin. Click "Yes."
So now you’re all set with the STL plugin: you can now export entire SKP files to STL or just specified groups. What about the 3D printer? It turns out that Make magazine released a great issue comparing a broad range of 3D printers so you can find the right one that fits your needs and your budget.
Credit - http://blog.sketchup.com/authors/tommy-acierno