When a medium character has readied an attack against a larger creature with reach and the conditions for the readied action are when the creature attacks the character I know that the character could make a 5' move and attack with their readied action. What I am wondering is can the character attack the large creature when the creature is using its reach to attack the character and the character doesn't move but the reach of the creature allows its natural attack/weapon to enter the characters square.
it sounds like your asking if you can attack the arms.
No, you need to attack the creature to attack the creature. You'd need to have not moved on your last turn and to be able to 5ft up to the creature to be able to hit them with your readied attack (assuming you're medium and they are at 10ft reach)