Tell him that you’re interested in Carts and Castle Gears.
[Ferrell Guild Merchant] Mildun will ask you to obtain 5 Black Hearts.
After collecting 5 Black Hearts, return to [Ferrell Guild Merchant] Mildun and tell him again of your interest with Castle Gears and Carts. He would then reward you with Drive Castle Gear Skill.
Now, you’re ready to drive a Castle Gear! Have a safe trip.