If you have not done so already, you must first actually download a version of the POP code, maybe even the latest version. The preferred place to get the POP code is our main web site at Los Alamos: http://climate.lanl.gov/Models/POP/

CESM1 Notes

If you are interested in running CESM1 POP2, you must download the entire CESM1 distribution from the CESM1 public release page.