Right-click an item in the Selection Tree, click Presenter on the contextmenu, click Shadows, and click the shadow casting option you require.
The available shadow casting options available for a geometry item are:
Off. Choose this to disable shadows. The selected item will not cast a shadow from any light source.
On. Choose this to enable shadows. The selected item will cast a shadow from any light source that has Shadows enabled.
Inherit. Choose this to inherit the shadow casting option from the parent item. That is, the selected item will use the same option as the item directly above it in the Selection Tree path (see Selection Tree Window for more information). For example, if you turn shadow casting On for a group, and the geometry contained within that layer is set to Inherit, then the geometry will also cast shadows, as it inherits the On option from its parent (the group).
Note: If all items in the scene are set to Inherit, then the default setting is On.