How to use PoE to power an additional device?
In PoE, you don't put a voltage to single conductors, but always to pairs. The standards allow two configurations: either pairs 1/2 and 3/6 OR 4/5 and 7/8. So, it sounds like this is a normal wiring. 


  Connected the step down module to the wires 2 and 6.


You can't do that: in PoE you need a center-tapped transformer to extract the DC offset that the switch puts onto the wire pair, and can't directly solder to the wires. 

The reason is kind of easy to explain: if you directly attach a load to either of the differential cables, you're just shorting the high-frequency signal through your load. 

A switch, by design, has to detect such a "signals shorted" situation and thus disables the port, typically. Even if it didn't do that, it couldn't use the port for ethernet anymore.

So, instead of doing what you're doing, you'll have to find a supply voltage inside your camera, and then you can attach your LED to that.