Very recently and for the first time the adsorption dynamics of a prototype molecule has been studied in detail on a metal oxide surface [Becker, Kunat, Boas, Burghaus, Woell, JCP 113 (2000) 6334], namely for the systems CO/O-ZnO and CO/Zn-ZnO, which both show a distinct increase in the adsorption probability with increasing CO coverage. This phenomenon (referred as adsorbate assisted adsorption) is in contrast to traditional precursor models. The presented Monte Carlo algorithm [submitted to Surf. Rev. and Lett.], which predicts the enhancement of adsorption by pre-adsorbates, assumes different adsorption probabilities for molecules scattered on bare and already occupied sites. Additionally, the effect of the mass-mismatch of the adsorbate and the surface atoms is considered. Thus, presented is an MC version of the analytical so-called modified Kisliuk model [e.g., R.J. Madix, et al., Surf. Sci. 470 (2001) 226]. In contrast to most of the analytic models, the MCS scheme includes lateral interaction energies, the influence of defects, and the effect of cooperative precursor dynamics. Although adsorbate assisted adsorption has also been observed on metal surfaces, the polar surfaces of ZnO are especially well suited to test the algorithm and the influence of the mass-mismatch on the energy transfer processes involved, since O-ZnO and Zn-ZnO differ solely by the mass of the atoms in the first surface layer. MC simulations will be presented which explain the main effects observed experimentally for both polar surfaces of ZnO.