How to install 500px?
500px is available in the official Kodi repository which is included by default in Kodi. You can install this addon by going in Kodi to System >> Settings >> Addons >> Get Addons >> XBMC.org Addons or Kodi.tv Addons
About SuperRepo and 500px
SuperRepo does not maintain 500px. We only provide an automated index for Kodi users to have easy access to 500px and other addons for Kodi. We are not affiliated with the developers (Patrick L Archibald [patrick.archibald[AT]gmail.com]) and do not provide help for this particular addon.