To access the RX/TX MAC you need to check "RS485 raw mac enable" in the device's advanced properties. This is documented in the help file (albeit a bit hidden, do a search on "rs485"). Also, from the documentation:
MAC address of the CobraNet Interface from which SCI data will be accepted. This may be any multicast address except 01:60:2B:FD:00:00 through 01:60:2B:FD:FF:FF, which have been reserved by Cirrus Logic for use as asynchronous global channels.
MAC address of the CobraNet interface to which serial data is sent. May be any multicast or unicast address.
The number of the receive channel to which this CAB belongs. This CAB will be able to receive serial data from other CABs transmitting on this channel.
The number of the transmit channel to which this CAB belongs. This CAB will be able to transmit serial data to other CABs receiving on this channel.
Essentially, the MAC values are useful if you want to explicitly choose the multi-cast address the serial data should be sent to/received from.