You may need to get your hands on CS3 for Windows to open the 5.1 project and then get a copy of CS3 for Macs to open the CS3 version, and then you might be able to open it in CS6 for Macs.
Might be cheaper/easier to start from scratch.
Thank´s Jim, why do not instead of CS3, I look for a CS6 copy for Windows and move forward from there on?
You can try it. I've never done this and my memory on the reports from others is sketchy, but I think I recall reading where such a new version of PP may not open such an old project file.