You must first load AC 15 Library:
File / Libraries and Objects / Library Manager... / Add... / select the folder
ArchiCAD Library 15 / pick the button Choose / pick OK.
Press the button "Windows" on the left bottom of the keyboard, then All Programs/Graphisoft/ArchiCAD 15/ArchiCAD 15, right click on your mouse and select Properties. In the pop-up window yo will see the full path of the program.