I'm assuming that this MacBook does not have a firewire port, so you're limited to USB.
An external soundcard for recording is usually called an audio interface, and you may have more luck searching using this term. I usually start with major site known to carry them, such as Amazon, Musician's Friend, Zzounds, or Sweetwater. There are many such sites, but these are some of the larger ones.
Since you've already been recording a mixed signal through a PC's line in port, I'm presuming that:
- You aren't concerned about recording multiple instruments to separate tracks simultaneously
- You also aren't overly worried about the quality of the digital converter (the majority of "line-in ports" on PCs are not famous for their fidelity or quality)
- This is mostly "for fun"
This means that you don't necessarily need more than two channels (since you're only recording stereo), and you don't need preamplifiers or XLR jacks, since you've already got the mixer.
Ordinarily, shopping recommendations aren't a good fit for this site, but given the assumptions above, I'd have a hard time not recommending a Behringer UCA202:
It not great (stero in and out, no preamps, so-so brand), but the one I have sounded better than any of my computers' line-in ports up to that point, and it's generally found new for around $30 US. It'll get you going for cheap and I can confirm that it works on both a G4 Mac Mini and on last year's Macbook Pro. It is class-compliant so you do not need to install drivers.
But if you want something a bit better but still in that range, here are also a variety of $100 interfaces available from many manufacturers with various improvements (better quality, better preamps, more features, whatever) but I can't really make a recommendation in that space that won't be irrelevant in a few years(and therefore not really useful on this site). I recommend going to any of the sites I mentioned above, or any like them, and browsing the "audio interfaces" section, sorted by price. Take the ones whose qualities you like and read reviews from people who have used them.
Once you've got an interface for the MacBook, the recording process should be exactly the same as it was on your PC.