The su command only works for the terminal session. What you probably want to
do is launch a program, probably the file manager. If you're on GNOME, the
file manager is Nautilus, so you want to use "sudo nautilus". If you're on
LXDE (Trisquel Mini), the file manager is PCManFM, so you want to use "sudo
pcmanfm".
You can also use gksudo instead of sudo, which is the same thing as sudo but
pops up a dialog box rather than asking you at the terminal (which allows
using it from a "run" window or in a launcher).
Alternatively, you can do the file manipulation directly in the terminal with
commands like cp (copy), either using them with sudo (e.g. "sudo cp") or
doing su beforehand.
