i had the same trouble with my intel graphics chip, du to missing drivers.
looks like you dont have a driver installed.
download and install them first (post question if you dont know what to get)
then get arandr (better GUI than lxrandr for extending a screen). in terminal, paste this:
wget http://christian.amsuess.com/tools/arandr/files/arandr-0.1.7.1.tar.gz
tar xzf arandr-0.1.7.1.tar.gz
cd arandr-0.1.7.1
and run via
./arandr
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you can also make a .desktop file for it. ( http://doc.slitaz.org/en:handbook:desktop#lxpanel )
i copied the existing lxrandr.desktop file, changed it to arandr.desktop and edited it to start arandr.
this automatically gives you a start-menu entry