I have seen some posts in other forums that people have been able to use an MX-850 with a Crestron Gateway and processor. Does anyone know how they were able to do this or know of a post that explains it?

You need a special .mxf or .mxd file for the MX-850, that contains the Crestron codes. If you search the Yahoo Crestron Programmer's Group's message archives for "MX-850", you can find all the details you need, including the file - http://tech.groups.yahoo.com/group/Crestron/