To install the MOPAC2016 executable, please read and follow the instructions in the file "Installation instructions.txt" in the ZIP file you downloaded from OpenMOPAC.txt.
The instructions for installing MOPAC2016 depend on the platform MOPAC2016 is to be run on, and may change from time to time. So instead of giving instructions here in the manual, it is safer to put the instructions with the executable.