Hi,
Life should not be that hard! Setting up the 3DNA environment has rarely been an issue, indeed yours is the first case I've come cross over the past few years.
I noticed that you set X3DNA as
export X3DNA=/usr/local/X3DNA/
i.e., with a trailing '/'. In the output error message, you have path
/usr/local/X3DNA//bin
Noticed the two '//' between X3DNA and bin? The error message means you have not put 3DNA bin directory into your command line search path yet.
Try the following in your current terminal, one line at a time, what did you get?
echo $SHELL
# do you get '/bin/bash'? -- verifies you have 'bash' shell
ls /usr/local/X3DNA/bin/find_pair
# this verifies that you indeed installed 3DNA under /usr/local
export X3DNA=/usr/local/X3DNA
# without trailing '/'
echo $X3DNA
ls $X3DNA/bin/find_pair
# If 3DNA is installed under /usr/local, this should print the full path
export PATH=$PATH:$X3DNA/bin
which find_pair
Please report back how it goes.
Xiang-Jun