you need to manually add the code to the xml file, but save it somewhere in a txt file or something, the MDX config software removes things it dosn't know about, so would clear this out each time you click save
2k1Toaster posted this in another link
You will notice that it is the exact same syntax as the other channels. Only difference is the type.
Code:<iochannel id="ChannelID" port="0" brain="MyBrain" type="digital_input"></iochannel>