transparency is a material property which you can set by layer by applying materials with transparency to the layers where you need them. Materials are shown in Rendered viewport displaymode only by default.
Alternatively, you can get transparency without creating extra materials by using the Ghosted viewport display mode.
Note that the first method using different materials allows to set individual transparency values for each layer, while the second method uses the same transparency for all shadeable objects defined under:
Rhino Options > View > Display Modes > Ghosted > Shading Settings > Transparency