As far as I know, the Bryce main window is permanently stuck on your primary monitor, sorry.
Edit > Preferences > “Use multiple monitors” appears designed to use your second monitor to display things like the materials lab while allowing you to still see your main window, which would have been handy except that there appears to be a problem on my system where the materials lab window is displayed halfway off the right side of the 2nd monitor where I can’t use it, plus Bryce crashes later when I do this. Perhaps someday that will be fixed, but for now I don’t use the “Use multiple monitors” option.
If you have too much time on your hands, you could research the options provided by your graphics card. I might be making this up, but I thought a long time ago I remember seeing an option somewhere in the Nvidia settings that allowed some windows to be displayed in a position other than their default to handle having multiple monitors, but I don’t remember the details or know if this could be be used in any way to fake out Bryce into going onto a 2nd monitor.
You can also check over in the Bryce forum ( http://www.daz3d.com/forums/viewforum/38/ ) to see if anybody there has additional information.