The error I get is that it is trying to load a part named plug.fem. However, in the set of parts that you sent, the .fem file is named plug_fem.fem.
You cannot rename plug_fem.fem to plug.fem because you already have a file named plug.prt.
Open the .sim file and OK the warning message. RMB on the .sim object (top item in the Simulation Navigator) and pick "Replace FEM...". Use the file browser on the Replace FEM dialog to navigate to plug_fem.fem, then OK the dialog. plug_fem.fem will now be the parent fem part for plig_sim.sim.
Note that the boundary conditions will be "asleep" because the fem part was changes. You will have to edit these and re-select the target objects.
Be sure to save after doing this.