SACDs generally have a CD layer in them, so your drive will play that by default. Most DVD-As have a DVD-Video component and that is what plays on your computer as well.
For either of these formats, you need to have an analog converter capable of delivering the high definition content. Those simply are not in Macs as of yet. SACD uses a separate form of encoding, so that is another issue, that the decoder would have to exist in hardware or software, which it doesn't on Mac. DVD-A requires higher quality converters, so even if your Mac could play it, it would be downsampled and dithered down to 48kHz/24-bit.
cpjakes