For MQTT to work in v8.1 Sp5 you will need to manually install the needed library files.
Please download the MQTT Library Files here: https://schneider-electric.app.box.com/s/j9faxnulp9f5wo8j3egim3tzcm7prv31
1. Stop and close InduSoft.
2. Extract the zip.
3. Copy libeay32.dll and ssleay32.dll to your InduSoft install directory\Bin folder.
4. Copy paho-mqtt3as.dll to your InduSoft install directory\Drv\API folder.