This is more an issue of the shared network paths rather than a FileMaker one. When you insert a file as a reference into a container field, it references it by storing the path. Considering that you have both Windows and Mac users, the path more than likely would be different.
Wish I had a Mac on the network to test, If so, I could probably give you a more specific suggested solution.
To expand on Mr Vodka's post, if you can mount a shared directory on both mac and PC's that have exactly the same file path, you won't have any issues like you do currently. I suspect that OS differences will keep this from working for you but could easily be wrong here. There are ways to set up a calculation field that computes different file paths based on which OS platform is currently in use and I think this is what you are trying to do.
A quick way to see the actual file path being used in a store by reference container:
Define a calcluation field set with a return type of text and simply enter the name of the container field as its expression. Place this field on your layout and resize it to 4 or more lines of text high. This field may be very helpful in debugging file path issues while you get your "calculated" container working.
Since all but the initial drive letter should be the same for both platforms, I think, you should be able to construct a calculation with this basic format:
Select "container" as the return type (drop down in lower left corner of specify calculation dialog).
I'd use the first, text calculation to compare the windows and mac filepaths and then see if I couldn't construct expressions that I can insert into the above calculation to come up with a container field that correctly references the file for both platforms.
Yes, I'm making progress by proceeding along the lines you suggest. I'll post back tomorrow when i have time to work through it more.