You may have found older threads pointing to the openHAB1 version, the last openHAB1 version available was 1.8 stable and 1.9 snapshot. The bindings name was correspondingly 1.8 or 1.9 for the snapshot.
That points to the fact you are using recent openHAB2 version 2.3 stable where all corresponding 1.X bindings (which are all compatible to openHAB2) are called 1.12. It should be fine to use.
Because it is a version 1 binding you cannot configure the parameters via GUI. You need to use a mqtt.cfg file.
Just follow the official docs for setting up your mqtt binding and here is another nice thread if you encounter any problems.
MQTT on openHAB is working great and very reliable. It’s fun to use 
Have fun.