Plex Channel for FlickR Photos
Locate your Plex Media Server Plug-Ins folder:
\\Plex\Library\Application Support\Plex Media Server\Plug-ins
I have a Synology Disk Station where Plex is installed as a Package, so the location might be different depending on which flavour of Plex you have installed.
Create a new sub folder called FlickR.bundle and copy the Contents folder from the GitHub repository into it. You might need to reboot or restart the Plex server afterwards.
If it doesn't work please have a look for the log in the following folder:
\\Plex\Library\Application Support\Plex Media Server\Logs\PMS Plugin Logs
There should be a file called.com.plexapp.plugins.flickr in which hopefully will be a clue as to the problem.