PLUMED can be incorporated into LAMMPS using a simple patching procedure. Patching must be done before LAMMPS is configured. After patching, one should enable PLUMED using the command make yes-user-plumed In the same way, before reverting one should disable PLUMED using the command make no-user-plumed
Also notice that command "fix plumed" should be used in lammps input file after the relevant input parameters have been set (e.g. after "timestep" command)
See also http://lammps.sandia.gov/doc/Section_commands.html for further info on processing LAMMPS input, as well as this discussion on github: http://github.com/plumed/plumed2/issues/67.
For more information on LAMMPS you should visit http://lammps.sandia.gov/