How does Huck ignore Jim's warning? Why is this significant?

I'm not sure exactly where you mean. In part 2 Huck wonders about the dead man, though Jim warns it's bad luck. A while later, Huck decides to go ashore and to find out what's new. Jim agrees but with the whole snake episode (Jim was bitten by a snake's mate when Huck's prank goes wrong) makes Jim weary of bad luck. Jim insists Huck disguise himself as a girl, with one of the dresses they took from the houseboat.