Try the instruction below to solve the error
1)verify the MTOMWse class is derived from Microsoft.Web.Services3.WebServicesClientProtocol in your mtom web refference proxy file --> 'reffrences.cs'. You will need to click "show all" button in solution explorer to view 'reffrences.cs'
2)make sure that mtom serverMode arrtibute is set to optional in your web service web.config file like follows..
<microsoft.web.services3>
<messaging>
<mtom serverMode="optional"/>
</messaging>
</microsoft.web.services3>