I cannot get my Canon 600EX-RT to flash using, my Rebel T4i.
i have changed the settings on the T4i, under Flash Control to Custom and selected the middle icon option.
the camera is in 'P' mode, and the flash is up.
on the 600EX-RT its in radio mode and slave mode... yet nothing happens..
The 600EX-RT can do (1) standalone, (2) radio master, (3) radio slave, (4) optical master, or (5) optical slave. But you must choose one of these modes. If it is in, say, radio slave mode, then it will ignore optical masters (it can be optical and radio at the same time.)
See chapter 5 of your 600EX-RT speedlite instruction manual. Specifically what you want is likely on page 78.
You'll press the zig-zag lightning bolt icon button (on the left) until the display shows the lightning bolt icon in the upper right corner of the LCD _and_ the LCD panel has the word "SLAVE" displayed.
Also, verify that BOTH the camera _and_ flash are using the same channel number. The idea behind multiple channels allows you to shoot in the presence of other photographers and hopefully not worry that their cameras are firing your flashes and vice versa (you'd ruin each others' lighting).