Chem3D Ultra (but not Chem3D) can run MOPAC jobs. Here are the instructions on how to install MOPAC2016 so that it will run inside Chem3D Ultra.
Before starting, make sure that there are no copies of MOPAC already on your machine. Search the "C" partition for files with the name MOPAC*.exe If you find any, then if its name is MOPAC2012.exe re-name it to MOPAC2012 (old).exe, or if its any other MOPAC executable, delete it.
Go to web-page: http://openmopac.net/Download_MOPAC_Executable_Step2.html
Download "stand-alone 64-bit MOPAC2016 for Windows". This is a zip file. Open the zip file, and using the instructions in the file "Installation instructions.txt", install MOPAC on the machine where you will be running Chem3D Ultra.
Run a test on stand-alone MOPAC2016 to make sure it works.
At the present time, Chem3D Ultra looks in the folder "C:\Program Files\MOPAC" for the files "MOPAC2012.exe" and "password for MOPAC2012" If these files are present, Chem3D Ultra will be able to run MOPAC jobs. At present, March 2016, Chem3D Ultra will not recognize "MOPAC2016.exe". To get MOPAC2016 to work inside Chem3D Ultra, make a copy of "MOPAC2016.exe" and re-name the copy "MOPAC2012.exe". Make a copy of "password for MOPAC2016" and re-name the copy "password for MOPAC2012"
If Chem3D Ultra is running, shut it down. Start Chem3D Ultra running, and MOPAC2016 should then run correctly. To verify this, look at the start of the output from MOPAC; it should say "MOPAC2016" and not "MOPAC2012"
If you only have ChemDraw or Chem3D, these will not run MOPAC, you'll need Chem3D Ultra.