MOPAC is a stand-alone program that can be downloaded from the internet, from web-page: http://openmopac.net/Download_MOPAC_Executable_Step2.html
Versions exist for all Microsoft Windows, Mac and Linux
operating systems.
They can be downloaded as a graphical installer that will prompt you through the installation of MOPAC,
or as an archive (.zip on Mac/Windows, .tar.gz on Linux) that can be expanded into a directory containing all essential MOPAC files.
Once MOPAC is installed, run a few simple
calculations. This is important! Only be running jobs can you
get a feel for the program. Of their nature, proteins are very large molecules, that's why they're called
"giant molecules", and all calculations involving proteins require a large
amount of memory. A minimum of 2Gb would be needed for systems of 5,000
atoms, but more memory, e.g., 4Gb, would be better. If several giant
molecules are to be run simultaneously, then 16Gb to 64Gb is recommended.