That Mani Admin Plugin tutorial is a little outdated. This is because it no longer runs using the admin list, it now uses a file called "clients.txt". This file can be created by creating the "adminlist.txt" file and placing your steam IDs in it like this:

STEAM_0:0:12345
STEAM_0:0:23456
STEAM_0:0:34567

Once you have restarted the server the "adminlist.txt" file will be renamed to "adminlist.txt.old" and the "clients.txt" flie will be automatically created. Please note that editing the "clients.txt" file can be tricky and mistakes will break the plugin so it is best to simply add admins in-game using the GUI.