The ionic form of the unbalanced equation
MnO2 +HNO2 = Mn(NO3)2 +H2O is as below
MnO2(s) + H^+(aq) + NO2^-(aq) = Mn^2+(aq) + No3^-(aq) + H20(l)
MnO2 does not dissociate since it is in solid state. In addition H2O is in liquid state thus does not dissociate in to ions