for mine (delete any commented out lines in txt editor (e.g. notepad or notepad++)
most important line in mqtt.cfg is
OH2MQTT.url=tcp://192.168.1.10:1883
my items look like:
Switch DO_70_4 “Ensuite Fan - manual override [%s]” (Ensuite,Node70,log) {mqtt=">[OH2MQTT:home/rfm_gw/sb/node70/dev19:command:ON:ON],>[OH2MQTT:home/rfm_gw/sb/node70/dev19:command:OFF:OFF]",autoupdate=“true”}
Number Tem_7048 “Roof space 7048 [%.1f °C]” (Roof, HouseData, Node70, All_Temperature,log,temp_chart) {mqtt="<[OH2MQTT:home/rfm_gw/nb/node70/dev48:state:default]"}
Number Tem_7050 “Bathroom temperature 7050 [%.1f °C]” (HouseData, Bathroom, Node70, All_Temperature,log,temp_chart) {mqtt="<[OH2MQTT:home/rfm_gw/nb/node70/dev50:state:default]"}