If you installed openHAB 2.1 stable using apt-get on a linux host (e.g. Raspberry Pi running openHABian), you should use PaperUI (which comes by default with the openhab package) to add options to your system (for example to install another User Interface like HABPanel)
Access PaperUI by browsing to: http://<IP_of_OH>:8080
Some useful info can be found here:
http://docs.openhab.org/tutorials/beginner/uis.html
http://docs.openhab.org/addons/uis/paper/readme.html
http://docs.openhab.org/configuration/packages.html