It may be that you have incorrectly connected the front case connectors and you have somehow shorted the Reset contacts so it boots and immediately resets. The fact that you also have no power light leads to the same conclusion. Look in the M/B manual and ensure they are correctly connected. There should be a diagram of the connections in the manual but sometimes they are so small they are difficult to read. To confirm just disconnect every connection except the Power on/of, which as you say just shorts two pins together to signal the PSU that it should start up.
Also check that the second PSU connector is well seated on the M/B.