Solved and working:
[System_X]
Name=Dolphin (Mario Kart GP)
WorkingPath=C:\Program Files\Dolphin 4.0-315
TablePath=C:\Program Files\Dolphin 4.0-315\ISOs
Executable=DolphinWX.exe
Parameters=-b -e "C:\Program Files\Dolphin 4.0-315\ISOs/MarioKartGP.iso" (THIS PATH POINTS TO WHERE EVER YOUR ISO IS. DON'T JUST COPY-PASTE THIS CODE. ALTER THIS CODE TO REFLECT WHERE THE GAME ACTUALLY IS ON YOUR PERSONAL PC. The same goes for the Working Path and Table Path as well as the Executable.)
Enabled=True
SystemType=0
Some notes here:
The posted solution above is SPECIFICALLY for anyone attempting to JUST add Mario Kart Arcade GP to Pinball X.
Before you do ANYTHING, you're going to have to screw around with Dolphin 4.0 (builds 309 AND 315) a bit. Yes, I KNOW we're at Dolphin 5 but, to my knowledge, the game ONLY runs on Dolphin 4.0, Build 315.
You also need the specific Mario Kart Arcade GP rom file. The only version that will work is the ISO version of the rom ONLY. I am not going to tell you where or how to obtain it.
If you have everything (both versions of Dolphin and the ISO), you're ready to rock. There's a pretty good tutorial here.
I am assuming that, like Daphne, you can't just add Dolphin and play all the games. You actually have to do a Dolphin entry for each game you want to play. Pain in the ass, but that's the way to go, I suppose. I could be completely wrong about this but I haven't found a code that would allow you to play everything, willy-nilly.
Anyhow...SUCCESS!