Radio Shack sells a small pre-amp that you can hook your turntable to... about $30Cdn. You do not need the pre-amp for cassettes from a cassette deck. If you have inputs, iMic is NOT required. you do need a miniplug to R/L RCA jack also from Radioshack.
The original question was the required app. I like Peak, but you do not have to go to that extent. Sound app is an extremly good shareware program. I have not tried it in OSX.
SparkME is freeeware that will record in OSX as well. You have to register and they will mail you a serial number. It is by TC WORKS. Not sure of the URL.
I hope this helps