Basically i did exactly what was posted at the beginning of this thread:
Opened Terminal, an then used the command
sudo launchctl unload -w /Library/LaunchDaemons/com.citrix.agadminservice.plist
That removed the "adminservice.plist" from my memory.
To use the "sudo" command in OSX Mountain Lion you probably must enable the "root" user of your Mac; per default root in MLion is deactivated:
http://support.apple.com/kb/ht1528
I did that, than used the sudo command (see above), and afterwards deactivated the root account again for safety reasons.
That´s it.
citrix still works on my Macbook, even without this agadamsinverice.plist. I´m not an computer or Mac expert so i can´t tell you exactly how Citrix continues to work without; as far as i understood, agadminservice.plist is a relict from a previous installation/version of citrix an is found only on Macbooks which are updated from snow Leopard or Lion (mine comes from Snow Leopard to Lion to Mountain Lion).
If you removed this agadaminservice.plist succesfully from your Mac and have problems using Citirx afterward i suggest removing Citrix and do a "fresh" reinstallation