Since the introduction of the unibody Macbook Pro 13, Apple has decided to omit the dedicated Line-In port. Granted anyone who actually uses a laptop for recording will use some kind of interface, sometimes you just need a quick connection.
The good news it that the Line-In port does exist, albeit combined with the Line-Out. Only one function can be accessed at a time. But it is just a simple drop-down selection in the System Preferences menu.
This will now allow your Macbook Pro 13 to accept Line-In sources from music players, pocket recorders, and mixers like the Mackie 402-VLZ3.