The file is not under administrator. In the top level folder of your site there's a folder plugins. Inside it there is a folder system. Inside it there is the folder admintools. I think you get the idea :) You just need to start looking for the file on directory above administrator.
I understand why you are confused. When you access Joomla's /administrator URL you are loading the administrator/index.php. This loads files located in subdirectories of your site's main directory (especially the plugins and libraries directories), as well as files inside subdirectories of administrator. Joomla is a complex application. What is loaded is not 1-to-1 related with the URL.
I hope this information helps!
Nicholas K. Dionysopoulos
Lead Developer and Director
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