How to access the same Vault on both Mac and PC?

I downloaded the software on both Mac and PC but the content I uploaded to the USB key while on my PC doesn’t show up in my Mac. When I open the software on my Mac with the password (same one on the PC) and double click on “USB Flash Drive” where I put the file from the PC, I get the following message “The Vault could not be loaded, it may be located on another computer or the original vault has been manually removed.” Well it wasn’t remove, I can still see the file on the PC.

Where did I go wrong ? How can I access the same files from both Mac and PC?