It's the big, shiny chest in the middle of the road in the Earthrealm VILLAGE at the beginning of Konquest. Not quite sure how you managed to miss that one.

...shocking.

that's cause it's not in C-3,
i had the same exact problem, it drove me nuts
finally just after walking around i ran into a chest and it happened to be him, so i looked at the map and if i recall correctly, it said E-4, but im not sure! it could be C-4 too
either way, it's not C-3, i don't know why all the guides say that