Hello, Pierre.
You can find an example of how to play a specific MP3 from the MP3 trigger using serial commands from a Mini Maestro in this post from Jon. Please note that along with the connection between TX from the Maestro to RX of the MP3 Trigger, you will also need a ground connection between the two boards. Also, you should double check that the MP3 Trigger’s initialization file is set up to make it expect serial signals. You can learn more about this initialization file and the MP3 triggers settings in the MP3 Trigger’s user’s guide. Once you have that set up, you can configure the Maestro’s serial settings to match (e.g. it sends the serial signals with the same baud rate).
By the way, if you are only trying to play one MP3, it might be more practical to trigger it using a digital output from a Maestro channel configured as an output, which can be done with the sequencer. According to the “Using the Trigger Inputs” section of the user’s guide linked above, when the MP3 Trigger is set up for using the trigger inputs, grounding the pin that corresponds to your MP3 filename should play that file. This can be accomplished by sending a digital low output from a Maestro channel to that pin.
-Brandon