The panel appears on the primary monitor. You have not said how you are setting your system up so I can't give you a very detailed answer. You will need to use xrandr to find out you current setup. In my case this is:
$ xrandr | grep -w connected
VGA-0 connected 1440x900+1600+0 (normal left inverted right x axis y axis) 408mm x 255mm
DP-3 connected 1600x900+0+0 (normal left inverted right x axis y axis) 344mm x 194mm
So, DP-3 is my laptop's screen and VGA-0 is my external VGA monitor. Since I want the panel to be on the VGA which is extending the desktop to the right of the laptop, I would run:
$ xrandr --output VGA-0 --mode 1440x900 --primary --right-of DP-3 \
--output DP-3 --mode 1600x900
The --primary switch sets the VGA screen as the primary monitor and causes the panel to be displayed on it.