It seems that you have not provided a configure file i.e., the script to run simulation. Download the tutorials from NAMD website in which you will find many example folders. Copy the configure file and edit it according to your need.
I also have a Linux 64-bit alongside, in which I tried NAMD. But the same message pops up. "No simulation config file specified on command line". I did download the sample config file for NAMD from the website and keep the file in the same NAMD folder. But, it is still showing the same error. I guess I am doing something wrong, but I am not able to figure it. Please explain to me about this.