I think I must be missing something but can't figure this out for the life of me. I am trying to change the slave groups on a pair of Canon 580EX flashes. I followed the instructions in the book "Press Zoom button until SLAVE flashes, then turn the dial to change the group. Only when I turn the dial the group stays at "A" and won't change. The same thing happens on both units so I don't think they are broken or anything.

Can anyone help me out?

press zoom until 'slave' is blinking then rotate the control dial to the right.
You should see 'b' show up etc.

Make sure you turn the dial to the right. Turning left won't change it.