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. Within Windows, there are two forms:
"Download 64-bit MOPAC2016 for Windows (includes a small run-time window)" This sets up a simple window, and is useful for running at the Windows level. Use this program for stand-alone jobs, it can run all but the largest proteins.
"Download stand-alone 64-bit MOPAC2016 for Windows" This program is intended for use within another program, e.g., a GUI, and is suitable for running all but the largest proteins.
Select and download the version of MOPAC you want. Open the ZIP file, and follow the instructions in "installation instructions.txt"
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.