Earlier on he mentions this is for a podcast rather than live music.
Yes I will be using it for podcasts, and sometimes the podcast will require more than 2 microphones.
Anyhow, as described before, you're best off using the XLR outputs rather than USB.
so, maybe purchase an XLR to USB converter cable?
Mackie mixers have a reasonable reputation though I haven't used any of their newer ones. The thing is, if your needs will only ever be two tracks, you might be better off with a simple audio interface rather than a mixer. Even if the mixer has multiple channels locally, the vast majority only let you feed two channels into a computer--you pan your mics hard left and hard right to do this. Besides Mackie, I'd look at Allen and Heath and Soundcraft (or, at the economy end of the market, Behringer).
What the mixer would be good for is, well, mixing your podcast live. You could have a couple of mics and maybe some kind of playback device for music and effects--but you be mixing them live BEFORE the computer.
The other thing to think about is whether you're sure you'll only ever need two mics. In my experience, no matter how many channels you cater for, what you need is always that number plus one!