That "add a circuit" looks like a very dangerous device to me. A 20amp fuse requires a certain gauge of wiring to handle the current safely. The wire gauge and fuse are a matched pair, to ensure the wire doesn't overheat or cause other problems. Unless you keep the total amperage between the fuses the same (for example, replacing a 20A fuse with two 10A fuses) then you run a real risk of causing an electrical fire on the wire from the battery to the fuse box. If a fire starts behind your dash, your car is DONE! And don't even tell me most people use it correctly. If the factory fuse says 20A, you aren't going to be able to easily use a 10A fuse on that circuit because the load probably draws nearly that much.
I would highly highly recommend not using that device. It would be FAR safer to tap into a wire after the fuse. At least then, the fuse will blow before your wiring is in danger of causing a fire.
Of course, you have to consider the load you are using. Does the MSD device say what the current draw is? I would think it isn't significant, maybe 1A? If so, choose a circuit that doesn't have huge power draws (avoid the rad fan, blower fan, radio, AC compressor, and rear window defrost, windows) and be sure to tap into the leg AFTER the fuse. It should be dead if you remove the fuse.
For larger loads, you can easily add your own fuse and wiring. I have a nice 1 or 2 gauge wire with a 40A fuse to power my stereo.