To compute the momentum density, first compute the selfconsistent charge density, and then do a band structure calculation. There, you specify those lines in the BZ along which you want to know the momentum density. To get the momentum density, you must add the flag momdens to the list of output_flags. Example:
begin pw_jobs pw_job scf pw_job band_structure end pw_jobs output_flags momdens
You also need to specify the Fermi level according to a previous scf run: fermi_level fermi_energy_in_Ryd
If you don't specify this correctly, you will get garbage!