You don't. It's for fusing the positive line.
You would take the negatives back to your power supply terminals, or some other distribution block based on what's needed.
For negative connections as well you're better off with 1 of my many other power distro boards like power8, power8 or psdist1/psdist6. The Hexfuse was basically made to fuse the 3 30A (max) outputs of power supplies when going into other boards that didn't have a fuse as the first thing on them. You can connect the negative wires direct to the power supply if you don't want to use something like a neutral block.