Train Instructions from Jenna!
From the luggage area at the airport, there will be signs for you to take the escalator down to the train. Keep following the signs to buy your tickets. You want to get to BEECROFT station. It'll cost you about $16.80 each (I know, they overcharge immensely JUST for the airport, I think it's disgusting, but unfortunately, so are toll charges on the freeway going to the airport >_> )
Next, get on the train going TOWARDS the city. You want to aim for Central station. It will be about 3 stops from the airport.
Once you reach Central, disembark.
You then need to find the platform for the Northern line (red colour) going to Beecroft station. SOME Northern line trains go to Gordon, DO NOT get on these. Go for the ones saying Hornsby via Macquarie Park & Epping.
There are lots of stairs in Central station and not many lifts. I tended to just lift my luggage wherever I could so I didn't have to travel all the way around the platforms to get to the lift.
Also, give me a call when you get to Macquarie Uni/Macquarie Park (it's a few stations before Beecroft) so I know when to drive down to pick you guys up from the station.