Hi Nazif,
It looks like you might not have the correct permissions to use clusters in Amazon EC2. Have you signed up to use clusters in Amazon EC2? You can do this at www.mathworks.com/ec2. What is the exact error that you see in the Discover Clusters UI? Can you copy and paste the text directly?
If you have an MDCS installation at your institution running MJS or HPC Server, then it should be discoverable via the discover clusters UI. I notice that you have a trial for MDCS - have you (or your sys-admin) set this up?
Thanks, Elwin