Silent shooting mode has to be set to “disable” if you want to synchronize an external flash with a 7D (as well as 5D MK2 apparently). It is a documented, but fairly obscure feature.
Live view gives interesting advantage for manual lenses. At 10x magnification it is possible to achieve very nice focus without having to change the focusing screen on the camera. I already wrote about that in the past, but I cannot stop enjoying it and this is why I am repeating myself. It is very good for macro, but if you are planing to use external flash, it is likely that your subject will not have enough ambient light on it. It would make you live view window dark. Here comes another obscure setting called expo simulation. If you set it to “disabled” the camera would use all the light it can to show you the live preview and will not take into account aperture and shutter speed that you have dialed for shooting with external flash. BTW, the menu item controlling exposure simulation is just above the live shooting mode menu item.
So, if you are to use external flash for macro and focus using live view magnification, disable the silent shooting and the exposure simulation. It would save a few hears on your head 
7d Menu Choices
