DX = Right Side
SX = Left Side
The Italian words for right and left must be similar to the Latin words "dextra" and "sinistra". You'll find similar markings on the buttons to be installed in ergo levers for use with an ergobrain computer.
As far as compatibility is concerned, there are 2 main types of brake pads currently available from Campagnolo. If my memory serves correct, the brake pads that will fit your centaur levers have a series of "V" shaped grooves in the braking surface. If they have diagonal curved lines in the grooves they will not work.
I'm at a friends house and don't have my bikes or parts with me so this is all from memory, but if I'm wrong somebody set us straight.