Ignore those instructions just download the zip file (scroll to the bottom of the page), but don't unpack it. Go to the ACP and head for Customisation/Editor/Toolbars (see below):
Once in there click the 'Add Button' button lol
Now you will see that you can upload the zip file you downloaded.
It will only take a couple of seconds to upload.
The button will appear in the tray at the bottom of the screen - just drag it onto the editor toolbar where you want it. Clicking on the button whilst in the editor toolbars section will allow you to set permissions as to where you want it to be used and who can use it.
You're done - next time you access the editor around the site the button will appear and you can add as many as you want to a page or topic.