Recently, a symbol detection scheme based on the Viterbi algorithm was proposed for binary Two Dimensional Optical Storage (TwoDOS) in the presence of Additive White Gaussian noise (AWGN). In this paper, we extend the symbol detection scheme to allow for multilevel modulation. We demonstrate the proof of concept in a typical non-linear high density storage channel with intersymbol interference and AWGN. The relative performance, between multilevel and binary modulations is presented, where multilevel modulation provides better performance in higher capacities.