Shared folder on one mac
I assume that by "I shared", you mean that you've granted other users permissions to read and/or modify this file. But that's all this action does: Granting permissions. It's only half the way, and usually the second in order. 

The simplest way (and the only one I know of) for the files to actually be shared between different users, is to have them in your "Shared" folder, which can be found in the "users" folder.



You would then access those file from this directory. For your own convenience, you can add this directory to your (left) Finder sidebar for quicker access.

There's a relatively recent and quite comprehensive article on sharing folders between users with this method over at MacObserver.