C:\Program Files\Geometer
but the person who installed it could have put it in any directory.
Double-click on the Geometer icon and once Geometer is running, if you issue the Open File command from Geometer, the file chooser will start in that installation directory. (If you double-click on a Geometer diagram, the file chooser will start in the directory of the file that you double-clicked.)
See File Chooser for details on how to use the file chooser.
Here are the usual subdirectories of the Geometer installation directory:
Example: If you are going through the online tutorial and it tells you to open the file called Circ.T, start the file chooser using the Open command under the File pulldown menu. Double click on the directory entry Demos, and then double click on the file name called Circ.T.
Example: If you are reading the reference manual and you would like to experiment with the file that it calls Ref1/Orthocenter.T, start the file chooser with the Open command in the File pulldown menu, double click on the Reference directory (that's where all the reference manual files are found), then double-click on the Ref1 subdirectory, and finally on the file called Orthocenter.T.
If you own the book, Geometry with Computers, its files are organized exactly like those in the reference manual except that you will first click on the GeomBook directory instead of the Reference directory.