Not a Mac user here but the principles would be the same whether you're on a Mac or a Windows PC. Basically you locate the folder where the image files themselves are currently stored, then you decide where you want them to be permanently stored, and then you move them to the new location. The article you linked to seems to give reasonable advice on how to do this but I would take an exception to part of the process he outlines. Angle suggests you move the files from the photo library onto the external hard drive. I'm a bit more paranoid - things can go wrong (rarely but it does happen) during a move or copy so I would do it in two steps, first copy (not move) the files from the source to the desired destination, then verify that all has gone well and everything came over intact, and only then delete the files from the original source location. Note that Lightroom allows you complete flexibility in where you store the original image files. You actually don't need to move them out of the location where they're currently stored in order to use them in Lightroom - LR allows you to import them into the catalog from anywhere while leaving the original image files in-place. All you need to know is where they actually are. So once the files are in the location where you want to store them, import them into the Lightroom catalog and you're done.
