MP3 Generation

I'm looking at MP3.com's software page, and I'm not clear on whether any of it
will let me do what I'd like to do. Perhaps some of you tuners have already
blazed this trail?

First of all, I'm working on a Macintosh, usually my G3 PowerBook, with no
special sound hardware. (I also have an ancient Mac IIfx with the original
Sound Accelerator card and DAT I/O, but I doubt if anything supports that kind
of ancient history anymore.) What I'd like is software that allows me to plug
my synth into the audio input of my G3 PowerBook, punch a record button on the
software, play the sounds, punch a stop button, then save it as an MP3. Simple!

Can anybody recommend such a program or system of programs? Thanks a bunch!