# run as root
apt-get install ejabberd
dpkg-reconfigure ejabberd

Confirm that the ports are not taken and that the server can connect to itself using the provided hostname.

The command netstat -tulpn is helpful to see used ports. If the application epmd is on port 4369, terminate it using killall epmd and retry with dpkg-reconfigure ejabberd. If the server is already configured, start it with /etc/init.d/ejabberd start.