To set up refractions, your geometry must have some an alpha less than 1. Specifically, the fourth parameter of the ambient command should be a value between 0 and 1:

ambient 0.3 0.3 0.3 0.1

refraction 1.5

Below is an example scene description:

add cam
name renderCam
position 17.845837 26.321945 -38.120468
camera lookat 15.155572 22.831070 -31.443937
up 0.015374 0.996323 0.084284


add grid 4
poly centroid
poly scale 60
ambient 0.2 0.2 0.2 1
reflection 0.1 0.1 0.1

add grid 4
poly centroid
poly scale 60
poly rotate 90 0 0 1
poly translate 80 0 0
ambient 0.4 0.1 0.1 1
reflection 0.0 0.0 0.0

add grid 4
poly centroid
poly scale 60
poly rotate 90 0 0 1
poly translate -80 0 0
ambient 0.1 0.4 0.1 1
reflection 0.0 0.0 0.0

add grid 4
poly centroid
poly scale 60
poly rotate 90 1 0 0
poly translate 0 0 -80
ambient 0.1 0.1 0.4 1
reflection 0.0 0.0 0.0

add sphere
m 20 18 0
diffuse 0 .1 0.1 1
ambient 0 0.1 0.1 .1
reflection .1 .1 .1
refraction 1.5

add sphere
m 0 18 0
diffuse .1 .1 0 1
ambient 0.1 0.1 0 .1
reflection 0.1 0.1 0.1
refraction 1.3

add light 
m -11 31 -17

add light 
diffuse .4 .4 .4 1
m 11 31 17