Ok,
dont mean to patronize you, but are you using an XLR-XLR cable?, is the red light above the main power light lit? if so it should work...could be a bad omni box...do you have a multi meter? you should be able to measure 48V across the pins, IIRC you should read 24V from each pin to the ground pin...im sure one of the wiser people in here can verify that...
hope this helps