3D waveguide in time domain.

Hi,
I am trying to model a 3D waveguide and I am using a Gaussian pulse and I need to define an excitation port for the model. I can do this in 2D, but it seems in 3D it is really difficult as far as I know.
I used the Lumped port but after computing the model, I am getting error saying that " Error in lumped port setup". I have been working to solve this problem for one week, but I am not able to do so.
I highly appreciate if someone can help me in this case.