Answer
$CH_3NH_2(aq) + H_2O(l) \lt -- \gt OH^-(aq) + CH_3N{H_3}^+(aq)$
$K_b = \frac{[OH^-][CH_3N{H_3}^+]}{[CH_3NH_2]}$
Work Step by Step
The ionization equation should describe the transfer of a proton from a water molecule to the base, therefore, the base and the water are the reactants, and the protonated base and a hydroxide ion are the products.
The ionization constant expression is based on:
$K_b = \frac{[Products]}{[Reactants]}$
* The water doesn't appear, because it is the solvent, and its concentration doesn't matter.