How to point Dropbox app to the existing Dropbox location

I can't understand exactly what you're asking, but if you want to have files show up in the Dropbox Public folder and also be somewhere else in the filesystem, it looks like the only option for now is hard links (on *nix operating systems, including OS X):

ln /users/cs/python/codes/1.py ~/Dropbox/Public/

That will create a 1.py file in ~/Dropbox/Public which will be another link to the same file on the filesystem, though it will look like another file. This means you can access the file the same way, and any changes you make to one will be the same with the other (since they're accessing the same location on disk). You can later delete the link in the ~/Dropbox/Public folder, and the other one will still work. Take note: if you delete one of the links and recreate the file there, they will no longer be pointing to the same location on disk.

On Linux Dropbox has a client (dropbox) and a deamon (dropboxd).

The client has the exclude command, which you can use to exclude directories. E.g. to exclude node_modules from Dropbox you can enter dropbox exclude add ./node_modules

dropbox help exclude will print the help information:

dropbox exclude [list]
dropbox exclude add [DIRECTORY] [DIRECTORY] ...
dropbox exclude remove [DIRECTORY] [DIRECTORY] ...

"list" prints a list of directories currently excluded from syncing.
"add" adds one or more directories to the exclusion list, then resynchronizes Dropbox.
"remove" removes one or more directories from the exclusion list, then resynchronizes Dropbox.
With no arguments, executes "list".
Any specified path must be within Dropbox.