The main board is supposed to be loaded with Canon's software. If you bought a board that does not have this software loaded you are SOL because only Canon can do that.
Even if you got one of those boards, the camera would still power up and show signs of life. It would something on the screen but you cannot do anything with it...but it would show life.
If you are not getting anything out of the camera then it must be a loose cable somewhere, or the DC power board is not working and supplying power to the rest of the camera.
Most power comes from 'normal' cables with typically black wires into a white connector. Most ribbon cables are signal only and not power. On the 450D main board I believe it is located on the bottom side of the board near the far right side, under a ribbon cable located on the top.