Thanks for using 3DNA! I am glad that you solved your two previous problems by browsing the forum -- that's exactly why the forum was created in the fist place.
While 3DNA is not a point-and-click package, setting it up should be straightforward and requires minimum effort, i.e., by setting the environment variable X3DNA and put $X3DNA/bin into your command line search path. To make life even easier for inexperienced Linux/Unix users, I have created a Perl script (setup_x3dna) to detect the SHELL and set up these two items automatically -- it will be included in the coming new release.
Now come to your question. The main output file name is derived from your PDB file by changing to (or appending) the ".out" extension. It will be in the same directory as your PDB structure file. Have a look at .inp file -- the 2nd line will tell you where the .out resides. Now I notice a bit of inconsistency here -- all other files are written in the current working directory (CWD), but this .out file is not. Maybe it is more logical to put the .out file in the CWD as well. I will make this change into the new release of 3DNA.
HTH,
Xiang-Jun