Unsure of syntax used in Searchable Encryption
$\mathcal{M}$ is the domain of the inverse trapdoor permutation $\pi^{-1}$. For example, if $\pi^{-1}(m)= m^d \bmod N$ implemented using RSA, then $\mathcal{M}=Z^*_N$. The domain/range of $\pi$ and $\pi^{-1}$ are all $\mathcal{M}=Z^*_N$. 

Generate a random big integer in $Z^*_N$ (roughly from range 1 to $N$) should be fine (there is a negligible probability of getting something not in $Z^*_N$, but since it is negligible we don't care).