Download the latest *.jar file from: https://jenkins.otr.mx/job/openhab2-unifi-binding/
Place it in your $OPENHAB_RUNTIME/addons folder (in Linux apt: /usr/share/openhab2/addons/)
Use discovery in PaperUI or manually define your things and items (see this thread for example configs)
edit: forgot the link to the UniFi Binding Readme for config info 