The "pedal box" is just a bracket under the dash at the top of the clutch pedal, it's what the pedal and master cylinder are attached to (it is directly behind the clutch master cylinder that you can see in the engine bay).
As for leaks, from memory, the fluid goes from the master cylinder, through a solid line to some sort of block at the bottom of the firewall on the drivers side then into a rubber hose to the slave cylinder. It's not a huge circuit so should be relatively easy to find any leaks. Start at the master cylinder and follow it down.