Can you play records? That is, can you plop an LP onto the turntable, cue the needle down onto the first track, switch your amp so that it does “phonograph”, and have the sounds from the vinyl fill the room?
Yeah? OK, then forget stupid iMic thing, unless your Mac lacks a sound-in port. For the rest of this discussion assume you do indeed have a normal sound-in port.
Go to the back of your amp. It will almost certainly have a place for leads for a tape recorder to record from (as opposed to tape playback). If you have a tape recorder hooked to it, unplug those plugs and set them aside until you’re done. These will most commonly be RCA jacks, one for each channel, yes? (If not, edit the rest of these instructions to accomodate whatever kind of hookups you do have back there).
Go to your Mac. Is your sound-in port a single stereo minijack sound-in port, similar to the kind of port you’d plug headphones into? I think some Mac models actually provided separate left and right channel ports, although that might have been for sound OUT, I’m not sure. The ones that did (if they did) might have consisted of two mono minijack ports or might have been RCA ports, I don’t know.
The easiest to hook up would be if you have two RCA ports (but that’s also the least likely Mac configuration): just buy a standard audio component cable, the kind that ends in left and right RCA jacks at each end. The most common Mac configuration over the years is probably single stereo minijack, so if you have that you need to either buy a cable that has left and right RCA jacks on one and and a single stereo male minijack on the other, or else buy the standard audio component cable and then get an adapter to adapt from that to single stereo minijack.
Anyhow, hook 'em together and then adjust the “play” volume on the amp so that on the Mac, when you monitor sound-in as your sound source, you’re able to boost the signal enough to pick up the softest sounds from the album, but not much higher.
The iMic thingie — is that the Griffin USB product I’ve seen advertised in MacWorld and MacAddict? You use that if your Mac was from the short era where Apple, for some inexplicable reason, dropped sound-in from the computers it sold. (I understand that they put them back in, that modern Macs have sound-in again.) If your Mac has no sound-in port at all, this is what you use instead. AFAIK it doesn’t do a damn thing for you if you already have sound-in. If you do indeed have and/or need the iMic, just substitute iMic in my instructions for everywhere that I spoke of the sound-in port.