You probably already have an answer, but if not I just wanted to assure you that it is simple to find a way up, so is worth just searching our yourself.
There is a track which leaves from a fairly distinctive section of the Furber Steps walk. You can also find your own way from a little further down the track if you are willing to scramble through a little scrub. The scenic railway runs down a natural fault in the rock. There is another break in the cliffs just to the east (some really interesting history about this area. The first coal mined from the Katoomba mine, a 200kg slab, was dragged up this gully by hand and exhibited in Sydney to get investment for the mine) and then there is Orphan Rock. From a saddle it is obvious where to go. There is a gate, but I have never seen it locked, or even closed, and the very rotten stairs remain. It is simple to climb to the top, and the old hand rails etc are still in place. Some stunning views from the top.
I first went there as a kid. Nothing has changed much since then. I've been back twice in recent years (one trip report here, with some pics from on top if you click through to them
http://fatcanyoners.org/2010/08/07/malaita-point-and-wall/) including at night. Given I found it at night, without following the foot pad, shows just how simple it is to locate if you are willing to look around.
Cheers
Tim