The answer you seek is as Winston Churchill famously said "a riddle wrapped in a mystery inside an enigma." You'd think that it would be part of the set up process or at least mentioned on that annoying "getting started" page. No, Zuckerberg had a better idea. Instead the feature is 3 layers deep in the settings, on a page that most users would never even think to visit. In order to access the feature a page admin must first click on the edit page button. Then once in the admin area, click the Resources link on the left side. Make sense right? In the resource area there is a link titled "Select a username". That's the one you want.
Now, you're probably wondering what a username has to do with a URL. In this case, this will be the part of the address after the domain name. Instead of a long ugly URL like this one -
You will now have a nice short address like this one -
Simple as that, but you'd never know it was there unless you knew what to look for. Now you know so go claim that name!