More information would be helpful such as what console you are using, Nintendont version, using SD/USB, SD/USB Model and size, controller being used, Nintendont settings. The more information you provide the more likely we are to help you out.
Sorry, I didn't see that your responded until now. Still can't get MK Arcade GP to load, so here are the details you asked for.
Using a US Wii U, Nintendont 2.260 (Wii U), loader files on SD, games on USB, SD is 2GB SanDisk "for Wii", USB is 750GB Acer connected with a Y-cable, using Wii remote+ Classic Controller Pro, Nintendont settings: everything "off" or "Auto" except for USB "on", Force Widescreen "On" and Maxpads "4".