The first method should work, assuming the texture exists. The command
Code:
env = viz.add(viz.ENVIRONMENT_MAP,'maps/sky.jpg')
will attempt to load 6 files named:
'maps/sky_posx.jpg'
'maps/sky_posy.jpg'
'maps/sky_posz.jpg'
'maps/sky_negx.jpg'
'maps/sky_negy.jpg'
'maps/sky_negz.jpg'
Ensure that these files exist. Check the resource toolbar on the left side of your script. There should be an item that represents the environment map. Expand it and it will notify you if any textures are missing.
If the textures exists and you still have this problem then your graphics card might not support cube maps.