If you need to display grid slice along the lateral (not along VS), you need to make this well active and select "Along the trajectory" in grid format tab.
Alternatively can create the grid slice in other seismic software and export it as X,Y and Z.
Then import X,Y and Z(!) to StarSteer as a polygon. You can then display this polygon on x-section window!
You should use this way if you need slice along the shape different from VS section or well trajectory.