Ideally, you need either a copy of FIleMaker Server or a 3rd copy of FileMaker Pro, though there are (less than ideal) ways to do this with just the two copies that you currently have.
The file first needs to be set up for sharing. In FileMaker 13, click the new tool bar button "Share". Turn on Sharing if this is the application that will host the file and then select your open file and the "all users" option.
Now up to 4 other users (if this is FIleMaker Pro) can use Open Remote to find and open the file if they have a separate install of FileMaker 12 or 13. Please note that FileMaker 13 features such as popovers won't work for your FIleMaker 12 clients and there is a setting in File Options that you may need to modify in order to permit access to the file by FileMaker 12 clients.
Ok so I've done that, and the pc with filemaker 12 shows my pc ( with FM13 ) on the hosts list, in the 'open remote' menu. When i click on the fm13 pc in the hosts, it doesn't show any files in 'available files' box.
If i view the remote menu from the fm13 pc itself, it shows the same pc on the hosts list and also the files.
Could you tell me what could be wrong here?