A better way to imagine the diagram you posted is to replace the battery symbol (the yellow and black battery symbol) with your DC fuse block. The positive end of the battery in the diagram is your light fuse connection. The negative end of the battery in the diagram is the negative bus bar of the fuse block. The load symbol (the black load symbol that confusingly looks like a 12V battery) in the diagram is of course your light.
Here's how I would debug the problem. First, completely remove the switch. Test the light by connecting the pos and neg wires from the fuse block to the light. If the lights works now then you know that's not the problem.
Now try the switch again. Run the positive wire from the fuse block to the switch center pin. Then the wire from the positive lead of the light to the other positive pin of the switch. Then run the two wires to the negative pin of the switch. One from the negative lead of the light and one from the negative bus bar of the fuse block.