Trouble connecting an electric guitar ?

[drbombay]

i have the newest Macbook, from late 2009/2010, with iLife '09. Now, on the apple website, it says that the headphone jack on the side of the macbook doubles as an audio input. However, when I connected my guitar to that by plugging my guitar into my amp, then from the headphones jack on my amp into the supposed audio input on the computer. I have also tried directly connecting the guitar to the macbook with an adapter from a standard guitar cable to a headphone or line-in type cable. The problem is is that when i open garageband and click input source, it gives me no option of line-in, whether on a real instrument track or an electric guitar one. I have found several tutorials, but these all being for iLife '06, so they didnt help much. If anyone could tell me how to fix this problem so I can just record my guitar, I'd be much obliged.