This is probably going to be a very useful thing to do, as I can imagine that lots of other Elonex ONE tips are going to start: "open up xterm and type the following commands..."
Anyway this is what you have to do to get xterm on your menu:
Pop up the virtual keyboard with alt-tab and hover over the xkbd logo.
Hold down the left button and select 'manual' and miraculously xterm will appear, running as "root".
type: cd /home/user
type: vi .icewm/menu
..then you're in the menu file. Find a suitable location for your menu entry, and type a line that says:
prog "xterm" xterm /bin/sh -c "xterm"
Make sure it lines up like the other lines, then save and close the file. (If '-c' doesn't work, try '-x' instead.)