Thanks for your answers.
Actually I configured cluster assigning real static IP to cluster. Now I am able to add cluster in the OnCommand System manager which is installed on my local machine where I have cluster node VMs, but I fail to add it on DFM server running in another machine. Here is the error log:
where 10.26.128.251 is my cluster's IP address
I guess still I have something incomplete in my setup.
I would be very grateful if you could assist here too.