There are a few rules that need to be followed in order to allow the markers visualization in the Streamer.
First, when creating the streaminfo, the triggers should be named 'Markers' and the type should be set to string.
Second, the markers cannot be visualized if you are not recording the data in the XDF (with the BDF file format you can visualize markers over the UDP protocol, even if they are not recorded). This is simply due to the different protocol that is used for the LSL and you cannot visualize the data prior selecting the stream. So, first you should select the stream and start recording, and then you should be able to visualize the markers.
Here is a Matlab® script, as an example:
disp('Loading library...');
lib = lsl_loadlib();
disp('creating a new streaminfo...');
info = lsl_streaminfo(lib,'boothl_Trig','Markers',1,0,'cf_string','Matlab');
disp('Opening an outlet...');
outlet=lsl_outlet(info);
disp('Now transmitting data...')
while true
outlet.push_sample(1);
pause (1);
end
Note that you need to add the LSL library for Matlab to path first.