Samplng Audio from Mac with M-Audio PreAmp and Garagband

I’m looking to use my 13" Macbook Pro, GarageBand, and M-Audio MobilePre to sample audio from Macbook. I know with the 15" and up Macbooks there is and audio in and audio out line that you can use a double 1/3 jack to do this task. When I try to use my preamp I get either a lot of feedback or low quality audio recording. Does anyone have any tips on how I can pull this off?

I just had a few minutes to mess with this problem and I got it to work. I had to set my headphone jack on my mac to input and in garageband set the mobilepre to output the headphone jack on the mac to input then run the 1/3 jack from the preamp out into to the headphone jack of the mac. Great sound and clear sampling.