If you bought your macbook before 3 days ago, then I'd recommend you buy a Firewire 400 external hard disk. If you bought the new macbook then you can only use a USB 2.0 external HD.
Either way, if you're installing Logic studio or Logic Express you must install the application on your boot drive (Mac) and you can specify to install some of the content (like Apple Loops) to an external HD.
Here's a link to an Apple support document that you might find very useful:
Let us know if you have any more questions.