T3i + YN-560 Problem

Hey guys... I am fearing the impending silly question here... it has dumb written all over it, but I just got the T3i, just got the flash, I expected to pop it in, tighten, turn on both, set T3i on "flash" and then pop and go... but flash wont go off.  I was able to earlier make the T3i built in flash work.  The YN manual states this should work exactly the same way.  Obviously not, so I know I am missing probably some mundane detail, like a zero somewhere to something... help!

Thanks......