The mqtt broker gives really limited messages. If you had parts of the format wrong for the subscribe method, it will probably just fail silently.
Read the documentation for MQTT closely to make sure the format is correct. I havent dont the simulink example, so I would reccomend you try mqttfx or the MATLAB based MQTT client (on FEX). Once you get the syntax correct using MQTTFX or MATLAB, its usually easier to make sure you enter the correct settings in blocks in simulink.
Sadly the RPI example in the thingSpeak doc has an outdated library (to do list), but it might give you a different starting point if the simulink version wont work.