You first need to run the license manager on the PC / Server where the dongle is connected. Once the license server is running you must enter that IP address on the client to check out a license. Information on how to connect the dongle and run the license server is available in the NetSim installation (Hardcopy or the PDF file in DVD).
Please check the connection to the server by pinging the server from the client machine. If the ping is unsuccessful, then it means there is a network communication issue which would need to be resolved.
If the ping is successful and NetSim still doesn't work, then it is likely to be an issue related to the Java path. Please see the article https://tetcos.freshdesk.com/solution/articles/14000040977-no-response-when-clicking-ok-button-in-netsim-license-window-