  We derive the decay widths for the leptonic decays of heavy charged
pseudoscalars to massive sterile neutrinos, $M^{\pm} \to \ell^{\pm} + N$,
within the frameworks involving the Standard Model and two-Higgs doublets (type
II). We then apply the result to $B^\pm \to \tau^\pm +$ "missing momentum" of
the Belle/BaBar experimental results, in order to measure directly the relevant
parameter space, including the mixing parameter $U_{\tau N}$.
