David - How you address the I/O is different depending on the platform you are using; however it is usually determined by the location of the I/O card relative to the other cards and then the channel number the device is wired to.  So for the SLC that we have used in many of our examples, the processor is always mounted in the far left slot, then I/O cards and specialty cards are allowed.  From that location we can determine an address that we can use in our program.  To determine the address you need to count slots from left to right.  So if we have an input card in slot 2, the first part of our address is I for input, 2 for the slot number and then (depending on the card) .0 or .1.  The .1 is only used for cards that have more than 16 bits of information.  Most common discrete I/O cards are 16 point cards.  The next bit of information we add is the channel number.  For channel 5 we add  /5. 

So our address we have created is I:2.0/5   
Note that we start counting at 0 so this is 0,1,2,3,4,5  (6th out of 16 channels)

For analog information (uses more than one 16 bit word) we usually only need to address the word level.  The beginning of our address (assume we put it in slot 3) would be similar, I:3.   The next number would associate with our channel.  I:3.0  would be channel 0 or our 1st input.  We can address the bits, but it usually is not necessary with analog.

I did this really fast so I may have put some confusing info in here.  Please reply back with any questions.