Macintosh - photos to cross stitch patterns?

I am trying to find a free trial download for converting photos to cross stitch patterns (using a Macintosh computer). I have found several, but once I download the trial I get a pop-up that says I need to choose an application to run the download. I'm not sure what to do at that point. Please let me know if you have successfully used any of these free downloads and how you installed them on your Mac. Thanks!