Focus: the Sigma cameras aren't particularly good at AF. They focus best in bright light, and on areas with contrast like straight lines or dark/light areas. I use centre point focus, and focus on what I want and then recompose.
File recognition: download your files using a card reader, not directly from the camera. As for the software, I am not aware of any problems with a Mac using Sigma Photo Pro as I use a PC. Sigma provides the latest versions of their software on their website, so you can try Sigma Photo Pro 4 if you have not already.
An alternative is to use Lightroom 3, which supports Sigma's x3F files. I assume you are shooting in raw, not jpegs. A Mac will definitely recognize jpegs.