Choose Tools Catalog from the Misc menu in the OPML Editor and click the Install link next to rssCloud.root.
In the OPML Editor, from the Windows menu, choose config.root and navigate to config.rssCloud. Open prefs. When you're ready to start receiving subscriptions and pings, set config.rssCloud.prefs.enabled to true.
If you want to have changes.xml maintained, set up your server for Amazon S3, and set enableChangesXML to true, and s3ChangesPath to the path of the S3 folder to contain the changes.xml files. The main file will be written every minute. There is also an hourly archive that maintains a history, in XML of sites that have pinged your server.