You don’t have to change anything on your system. You’ll have to understand, that the MQTT binding is a 1.x binding and as such needs configuration via config files rather than through Paper UI. Read more about it here:
Everything you need to know MQTT specific is described here: http://docs.openhab.org/addons/bindings/mqtt1/readme.html
An example about how this could look like with an example device:
- ITEAD Sonoff switches and sockets - cheap ESP8266 Wifi+MQTT hardware
- https://github.com/arendst/Sonoff-Tasmota/wiki/openHAB
Especially while following along the last link, you will realize that this is quite easy! Good luck!