I have the plug but when i try and play it only records thru the mic on the mbp and not thru the pickups. Is there an option im missing?
You need to adjust your selected audio input and output signals in two places.
In garageband:
GarageBand --> Preferences --> Audio/Midi pane.
Also, in GB, when you are looking at the details of your Real Instrument (i.e., your "amp"), try setting the Input to Channel 2 (mono), and turn the monitor on.
Next up,
In your applications folder, in your utilities folder (in the default setup), open your Audio/MIDI setup app. Here you can change your audio input and output also.
iirc (it has been a while), I would plug my guitar into my M-Audio box, and that into the Mac. Then I would make sure the M-Audio box was selected in the Audio/MIDI setup app as my input, and I would chose an appropriate output. I would then boot GB, hop into the preferences and make sure my audio input and outputs were selected properly (usually matching whatever I put in my Audio/MIDI setup app), and then I would configure my amp. If you are still having trouble hearing yourself, try switching your amp settings to channel 2 (mono), and make sure monitoring is turned on.
You should be good to go.
Finally, unless your guitar has active electronics, which boost the signal from the guitar, you really should pre-amp your guitar into your Mac. I don't think this is your problem, as you should still hear something from a guitar straight into GB, but you won't get great tone until it is pre-amped.
Cheers,
James