I've also tried that (updated the question with a screenshot), when the node is configured for "Convert betwee, JSON String & Object" or "Always convert to Javascript Object", i get the following error:
"Unexpected token M in JSON at position 1"
In the screenshot above, I have it configured as "Always convert to JSON String".
The string the mqqt node is sending is not a correctly pharsed json string, it is missing all quotes. Do you have control of the json that the mqtt node is receiving?
I do have control over the JSON sent throught the broker to Node-RED. However, because we are developping in parrallel I am testing the Node-RED by sending direcly from my terminal on the machine hosting the broker.
When you say missing the quotes, which ones are you reffering to? (because I already have some as shown in the question in the object)
What you could do is get an inject node and enter the message there - that which is being sent form the device - and put that through a JSON node then a debug node and set the debug node to show what you want.
Then, you tweak the message being injected until you see the right message.
Having done that, make the code on the remote device send that message.
Ok, he beat me to it.
Put that message that @E1cid just showed you into an inject node (parallel with the MQTT IN node.
Ok, so I now get the dropdown that you mentioned but the same undefined message for the debug node trying to extract the MAC_Address... Is my javascript wrong?