Extending John's remarks, when you switch to Slave, you may need to hold the mode button down for 1 second to force the flash into Manual mode. Then press the center button and turn the wheel to select the power setting (1/1 full manual or any other output.)
The wireless transmitter will fire the flash in manual mode *but* it will still fire its own infrared preflash. That makes it impossible to add optically-slaved flashes to this setup (which I find annoying.) But it will work fine for any number of Canon flashes set to Manual mode.
Also, you can't shoot in High-Speed Sync mode while using Manual flash.