OH is probably a bit heavy weight to serve merely as an MQTT bridge. Is there not already a Niko2MQTT script out there on GitHub or something? OH is a hub. It’s designed to be the central point of control for all your home automation. It’s going to be far more complex than is warranted if all you want is to bridge to MQTT. OH is a full platform and you have to do a lot of work with the docs to get to where you can implement something like this in OH.
That being said, Marketplace MQTT Event Bus is probably your best bet for simple. However, deploying it requires a fairly good understanding about how openHAB itself works. If nothing else it will provide another example.
But there are no details in “can’t get it to work” so 
. Logs? Errors? Does the broker Thing show as ONLINE?