The polyethylene.emc file in the lib directory is a native EMC input/output format and holds the crystalline structure for united atom polyethylene. It stores positions, force field parameters, etc. It is meant to be used in conjunction with an EMC script. One can write these EMC scripts themselves, or use EMC Setup to generate one. The latter would generate an EMC Script file called build.emc. One can use emc_template.sh to generate an EMC Setup suggestion for importing by
emc_template.sh import [name]
The latter will write a file called name.esh or setup.esh when name is omitted. A more complex EMC Script example for polyethylene can be found in
examples/build/polymer/polyethylene/build.emc
The latter is a suggestion for how to create input structures for united atom semicrystalline polyethylene.
Regarding your second post
You trying to import a polyethylene crystal structure, which already contains group definitions for the methyl group, which is using EMC not as intended. You do not have to import the polyethylene crystal structure, if you intend to only build amorphous polyethylene. I would suggest using
examples/build/polymer/polyethylene/build.emc
if you are interested in generating semicrystalline polyethylene.