There are two plugins here. Download the ZIP archive of FredoScale as well as LibFredo (under the other link). Extract both archives to your Plugins folder. Restart SU. Then open your Preferences window (Window menu) and under the extensions tab, activate FredoScale. Now under the Tools menu, you should have a "Fredo collection" item with FredoScale in it. You can also activate its toolbar from the View > Toolbars menu (Make sure to save your toolbar position first under the same menu item)