Turing reducible in natural numbers?
To show that $A$ is Turing-reducible to $B$ you need to prove the existence of a Turing Machine that is able to decide $A$ when given access to an oracle for $B$.

In your specific case a possible Turing Machine $M$ takes as input a string $x \in \{0,1\}^*$ encoding the natural number $n$ (I'm assuming $0 \in \mathbb{N}$) in binary and operates as follows:


It flips the last bit of $x$. Now $x$ represents an odd number iff the input number $n$ was even.
It invokes the oracle for $B$ with input $x$.
It accepts iff, according to the oracle, $x \in B$.


Notice that the fact that $M$ has access to an oracle for $B$ does not mean that $M$ must use that oracle. The following is also a valid choice for $M$:


Locate the last bit $y$ of the input string $x$.
If $y=0$ accept. Otherwise reject.