Submitting an interactive job with qmon doesn't work

This is because the node that the job gets submitted to cannot open a window on the machine you work from. To grant all nodes access to your workstation's graphical environment open a shell (a window with the unix prompt) and type xhost +<RETURN>.

Note however that this is a safe thing to do only while you work with the cluster, but not anywhere else : xhost + gives access not just to the other nodes, but to each and every computing machine on this planet. Consider using (instead of qmon's 'interactive job') the command qtcsh (just type qtcsh<RETURN> on the unix prompt).