  We consider a $U(1)'$ supersymmetric seesaw model in which a right-handed
sneutrino is a thermal dark matter candidate whose relic density can be in the
right range due to its coupling to relatively light $\tilde{Z'}$, the
superpartner of the extra gauge boson $Z'$. Such light $\tilde Z'$ can be
produced at the LHC through cascade decays of colored superparticles, in
particular, stops and sbottoms, and then decay to a right-handed neutrino and a
sneutrino dark matter, which leads to lepton flavor violating signals of
same/opposite-sign dileptons (or multileptons) accompanied by large missing
energy. Taking some benchmark points, we analyze the opposite- and same-sign
dilepton signatures and the corresponding flavour difference i.e., ($2e-2\mu$).
It is shown that $5\sigma$ signal significance can be reached for some
benchmark points with very early data of $\sim 2$ fb$^{-1}$ integrated
luminosity. In addition, $3\ell$ and $4\ell$ signatures also look promising to
check the consistency in the model prediction, and it is possible to
reconstruct the $\tilde{Z'}$ mass from $jj\ell$ invariant mass distribution.
