OK, well, you'll need a MIDI interface to get the notes and controller messages from your keyboard into your Mac, and while it is possible to get the audio out via the headphone socket, I wouldn't recommend it in this instance, so you will need to get yourself a standalone interface.
The good news is that most audio interfaces also have a MIDI interface built in. You basically attach the interface to your MBP by firewire or USB cable (depending on which interface you choose), then hook up your keyboard to the MIDI 'in' socket on the interface, and take the audio out to the PA via the 'out' jack socket(s) on the interface.
There are many interfaces on the market, and in choosing one, you need to decide your price range, how many ins and outs you need. If you are only planning to use a keyboard and nothing else, then technically you only need a stereo out pair, but do consider what you may need in the future, for example, if there's a chance you will want to bring audio signals into your Mainstage or Logic (ie guitars, or vocals). Also, for the number of outs, consider whether you will want to output just the main signal that will go to the PA, or whether you want to set up different mixes of sounds (eg for monitoring). If the latter is the case, then you'll need an interface with more outs.
You'll also need to consider the sound quality, and generally speaking, you get what you pay for in this respect, so think about whether you'll just be using this set up for live performance, or if you also want to use it for studio work (in which case sound quality is more critical).
Personally I use a MOTU Ultralite, which I've found to be a very good and versatile unit, but if you search this forum, you'll find lots of discussion about which is the best to use.
Hope that helps
