Looking at an astro forum, I think DSS may be Windows only. Google DSS for mac and you may find a workaround, but it seems not many have managed to get it to work. There's Registax which I think is also free. I'm a Windows user so I don't know if it runs on Mac. I used DSS for a number of years till I bit the bullet and bought a dedicated astro software package, but you don't want to be going down that route if you don't have to! You might be better off finding someone with a Windows machine you can borrow or use. DSS is pretty simple to use and quick, though I would only use it for stacking. Export the stack as a tiff and process it in your usual pp s/w. It autosaves the stack so if you make a complete hash you can go back and re-export it.