First of all you need to be registered as a Rise Developer. This is a one click process, go to Edit Profile, API key and click to "generate an API key". Once this is done you are registered as a Rise Developer, you are also automatically promoted to Bronze access which will give you access to more Rise features.
Now visit the Share page for your Rise Board. A new format will now be available "XML" which allows you to download the Leaderboard in XML format.