You may want to read up on how nam, and related tools, actually work ;)
I'm not very familiar with those tools, but according to the manual you need to create a trace file that you then can hand over to nam:
The first step to use nam is to produce the trace file. The trace file contains topology information, e.g., nodes, links, as well as packet traces. The detailed format is described in the section 49.1. Usually, the trace file is generated by ns.
The tool ns is in a separate package called ns2, so you may have to install that additionally.
According to this tutorial you need to write a tcl script that describes the network you want to simulate. Then, you feed that script into ns, which generates a trace file. And then, you can feed that trace file into nam.
ns, nam and related tools are scientific tools which are often not very big on usability. So you need to write some code, learn some syntax and generally learn how the tools work. There's not much hand-holding offered, and the user is expected to read up on things on their own. Maybe you're more used to programs that are targeted to the general public, which often offer much more direct help and intuitive operation.
