I'm not quite sure what you're asking...
Before doing any wiring, please make sure that your trailer is not plugged in and that you've disconnected your battery.
You access the fuses (and add a new circuit) at the front of the box, not the back. When you open the fusebox door from the front, there are two large screws near the top on either side. Unscrew these screws. Then, grabbing the plastic cover at the top, tilt it forward and it will easily remove.
Run your 12v line from the back in with the existing 12v wires. Select an empty spot on your fuse panel and screw in the bare end of the wire. For the ground wire, there are several grounding blocks screwed to the floor behind the converter - pick and empty spot and add your wire. Add an appropriate fuse to the fuse panel, reassemble everything and you're good to go.