The problem was in Rockwell's RSLogix 5000 software. In RSLogix 5000, tags (variables) can be setup to have "External Access". There are three options for "External Access" to either None, Read Only, or Read/Write. If the tags are setup as "None", the HMI will not be able to communicate with the PLC. You need to set the tags up for "Read/Write" in order to get communications between the Magelis and Rockwell PLC.