I assume the Shure Super 55 is XLR? (on the bottom it will have 3 holes for a cable to be plugged into, pics on bottom of post) If that is the case then the guy at best buy is an idiot. you will need to buy an audio interface with an xlr input. Usually you can find a cheap maudio brand one for 50 bucks or so online or at best buy. The interface plugs in via firewire or usb and the mic plugs in via xlr cable into the interface.\
So basically it goes, MIC>XLR CABLE>AUDIO INTERFACE>USB/FIREWIRE>Computer
If this mic is for some reason a usb then you simply just plug it into your macbook. I would recommend however that all you need is a Shure sm58 which is 100 bucks instead of that expensive mic you bought and is very directional. With the money you save you could get an audio interface, stands, or a editing program.
If anyhting I said doesnt make sense let me know I will try and explain it better.