As the error message explains, the OpenFlow component is unable to open a listening socket because something else is already listening on the same port. It's probably another controller. You might check that you don't have another controller running with the default port (which is 6633). There's also a small chance that some other non-controller is using the address.
You could also just specify a different port for POX to run on and then give that to your switches to connect to. As it indicates, you can do this by using the --port argument to the openflow.of_01 component. For example: ./pox.py openflow.of_01 --port=6655 You can find out more about this in the POX manual.
You may also find information on this subject by searching. For example, a quick Google search brings up the following related pox-dev mailing list post: https://www.mail-archive.com/pox-dev.noxrepo.org/msg00563.html