NS2 PROBLEM: Where to type codes?

Open any text editor and key-in your ns2/tcl simulation script and save it with extention “tcl” like experiment1.tcl.

After that, you can run it from the terminal window ( here we assumed that ns2 is already installed in that machine)

# ns experiment1.tcl

Writing C++ level agent or modifying an existing agent/protocol and compiling ns2 is little bit difficult than that of writing and running a tcl level simulation script . So, you can not get any simple “step by step” answer for your second question. Learn it from appropriate tutorials.

You may start learning it from the following link
http://www.projectguideline.com/modules.php?name=Forums&file=viewtopic&t=13