Looking at the Lua, it looks like all you need in your macro is /console stna, as the addon itself instructs all alts via the send command already. I think your macro right now is instructing your alt to execute stna (via send), but that alt isn't afflicted by anything, hence the message.
Hope I'm making at least some sense!
Thank you very much!
I tested and it works (most of the time).
Sometimes, it gives me a message that I am not afflicted (even when the main character is definitely silenced), so not sure why. I tested it at The Mute in Beadeaux so perhaps it is something to do with getting silenced by the device rather than a mob...
I will try it out some more in real battle situations.
Again, I appreciate your help!