Thanks for using 3DNA and for posting your questions on the Forum.
There are two parts to your questions, related to 3DNA per se, and do_x3dna on top of it. You have installed 3DNA successfully since you can access
find_pair or
analyze. I am not sure you have installed do_x3dna correctly since you cannot run it on a trajectory file.
Rajendra Kumar, the developer of do_x3dna, may be available to answer your questions on the Forum. Or you may directly contact him for help.
As to "set the environment variable X3DNA and add $X3DNA/bin to your command search path", the post you referred to has explained it all -- pay close attention to steps 4.4 and 4.5 which has annotated screenshots (see the links below). If that does not help, please consult a local, experienced Windows user.
and
HTH,
Xiang-Jun