Configuring the Update Manager to find the Buckminster update site requires you to fill in two fields in a graphical wizard. That is often simpler then to download an XML file on your computer and then have the Update Manager import that file. At present we also publish the subclipse (needed only by those who wish to use Subversion) plugin on our update site.
I think the installation instruction should describe the most common thing first (i.e. simply add a new site to the Update Manager using the UI) and then mention the ability to import the XML document as an advanced alternative.