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No shadows on Prelude II base pad:
Prelude II is a vessel and can't receive a shadow. The big advantage of having a vessel is that you can spawn them everywhere with the scenario editor and don't need to edit the base configuration of Orbiter with a base that would exist in all scenarios. Second advantage is that in contrary of base, the textures are filtered and look way better. Third avantage is that you can have a DLL and so an active base. Fourth avantage: as ships are not exactly over ground (see above) the shadow would look a bit off.
The drawback is that you don't have shadow on pad. One possible workaround would be to remove the pad from Prelude II (base's menu allow that) and declare the pad's mesh as a base using usual Orbiter config file. You'll just need to place the « Prelude II » vessel exactly over this pad and you would have both advantages: a vessel and shadow on ground (with textures not filtered, one can't have everything) (Note: I planned this workaround for Symphony IV the « big brother » project of Prelude II that wasn't released due to time, but I didn't test it on Prelude II) The meshes of pad are located in Meshes/DGIVPreludeII/PreludeIIPad.msh or Meshes/DGIVPreludeII/PreludeIIPadMountain.msh