Connecting Magelis to SY/MAX

I am using a Magelis (XBTF034510) with a TSX MBP 100 connecting modbus+ to a NR&D MEB-RT card rack addressed at 30-155 in a SY/MAX with a 650 processor. I'm having no trouble reading the mailbox registers on the Magelis however, I cannot write variables from the Magelis to the mailbox registers. I get the fault "impossible to write variables". Am I doing something wrong or is it not possible for Magelis to write variables to an MEB? Using a dialog box, the Magelis will write system data such as last page viewed, comm control and checksum to the MEB-RT, but not the variables associated with touch zones. Any help with this would be much appreciated.

Thanks,
Randy Lewis