Any addons used in a series or event is packaged in an easy installer and organized under the "Files" tab on each series or event page.
All Race2Play files are self extracting executables, so two simple steps are required:
All 'addon' tracks and modifications were created by their respective owners/creators. Though the 'addon' may have been re-packaged into an easy installer, no other files have been changed. Any released readme.txt by the makers are included.
Alternate location: http://files.race2play.com