The standalone "lmg" is deprecated and has not been updated for some time (in IC5141) (and now not shipped with IC61 releases).
Instead, place an instance of analogLib/mtline on the schematic, and change the "Type of input" to "Field Solver". You can fill in the parameters on the form (the same as you did in lmg in the past), and can hit the "Display Cross-section" button to visualize it (using the same kind of UI as you had in lmg).
The idea is that the field solver is run by the simulator rather than it being a standalone process. The benefit of this is that it allows you to parameterize the physical dimensions and then do things like parametric sweeps of these parameters. That was not possible if you had to run a standalone field solver.