On a Mac, you can ...
- drag the file icon onto the application icon (in a Finder window or the Dock)
- use a right-click on the file to specify the application to open
- use the Information dialog on the file to do the same and save that setting
Under Windows, the same or equivalent actions should be available.
I'm not understanding the suggestion. FM15 is in the dock.
The workaround here is to explicitly open the file with a specific application (version), using one of the suggestions given above.
If you just double-click the file, OS X / macOS will try to open it using what it considers the default application for that file type, in this case FMP12, since it was installed first.