I am surprised some other S owners have not come along to assist as I know there are a lot on here who do not have the radio controls on their steering wheel.
Tapping the i button (not holding it should cycle you through l/100km, trip, distance to go, time and "i". When you reach "i" hold it for 2 secs and you should enter the sub menu (your on your own then). Holding the "i" when on l/100km for more than 2 secs will reset the message centre.
Why not try posting on the DiscoUK site - someone there should be able to help.
DISCO3.CO.UK - Index
Garry