You need to map Commands 01 for left and Commands 02 for right, should be in the C section of the controls tab if you're on 0.37, if you're using 0.4 and up you can change the controls in the input.map folder in Documents but it would be easier to use the controls tab in the 0.37 configurator and then copy and paste the input.map from that Documents/Rigs of Rods folder to the Documents/Rigs of Rods folder for 0.4