1. Here's how

Please note that the following technical constraints apply:

• A PureGlass Material will only cast shadows when it is lit up by the sun.
• Artificial light sources will not make objects with PureGlass Materials cast shadows.
• The standard Glass Material cannot cast shadows at all. 
• If you use colored PureGlass materials, the shadows will not be tinted by the glass.

1.1: To get a glass object to cast a shadow, make sure that the sun is shining on the object and then assign a PureGlass Material to the object. This material type is available on the Indoor, Outdoor and Custom tabs in the Material Editor:

1.2: There are 16 levels of transparency/opacity for PureGlass shadows:

Set the Color Selector to less than white to affect the level of shadow:

You can set the Transparency (Opacity in versions prior to Lumion 10) and the Colorization slider to any value for the look of the glass.  

1.3: Note that PureGlass shadows are only visible when you render a Movie in 4 Star Output Quality - or when you render a Photo in Photo Mode or via the Current frame tab in Movie Mode.