Set your camera to 0,0,0 (so it is facing towards the top of your Bryce world from the top down view). Now click on the word sky to bring up the sky tools. Click on the icon of the sun to switch it to a moon. Now move the light gray dot to the top edge of the half sphere that makes the sun/moon move. Click on the rainbow to go to the sky lab. Follow Dreamshaders instructions. Also go to http://www.bryce4all.com/tutorials.htm and read the custom sky tutorial. It might help.