If you created the route/flight plan using the default P3D4 Flight Planner, you save to the Documents\Prepar3D v4 Files folder.

If you created the route/flight plan with the 757 CDU/FMC (usually referred to as the CDU), then you save it from the CDU (after activating the route/flight plan), and it will be saved in the I:\Lockheed Martin\Prepar3D v4\Captain_Sim\757-III\routes folder as a Captain Sim .rt file, that can only be loaded using the 757 CDU.