It's true, you have to put forth a little effort to access What.CD, but only because they take quality very seriously. The fact is, many MP3s you download out in the wild have been transcoded (converted from one lossy format or bitrate to another). This is especially true for downloads marked as 320kbps which seems to be a "sweet spot" for a lot of downloaders. What.CD does not allow transcodes, and for good reason - transcoding degrades audio quality.
For the interview, all you need to know is right here:
What.CD: A Beginner's Guide
To keep your account in good standing, you just have to give back what you take.
And regarding transcoding FLAC on a MAC, a quick look at the What.CD forums suggests that XLD is the application to use.
X Lossless Decoder: Lossless audio decoder for Mac OS X