I just wanted to give an example of another way of going about changing the file that you want to edit. I found it easier or quicker.

Install rootsh and leafpad from the extras repo(leafpad is really easy to save and edit as opposed to vi or vim)

This is an example for giving the extra options for the power button.

Open xterm
type: sudo gainroot
type: cd /etc/systemui
type: cp systemui.xml systemui.xml.back
type: leafpad systemui.xml

leafpad will open in a new window. It is just like using notepad in windows and scroll down till you find the line starting with:
<!--
and delete it and then find the line starting with:
-->
and delete it and then click on the title bar and click save and then reboot the device.

All credit goes to the original poster, I just wanted to share maybe an easier and quicker way to do it with terminal commands and a simple text editor.

If you uncomment it all it also gives you a button for the top command to open up in a xterm window and also a soft poweroff option.

Also wanted to mention pwngu!n had already posted the quicker way using vi and I missed it when scanning through the thread the first time.