The ball in question goes into the slave, under the bleeder. The bleeder should have a flat internal profile, not tapered like the restof the brake bleeders. OR it could be the ball from the clutch microswitch... prolly the only 2 possibilities.
Nothing else would have fallen out. I am unfamiliar with the MPH kit... I would think that if there was a leak, you'd see fluid eventually, must be air trapped somewhere...