I just got the slave today and started playing around with a few settings..
Here's what I came up with so far: I'm using a D70 to trigger the Optical slave. first I just put in on and took a few shots and I had the popup covered with the 540 so it had no effect on the outcome. The photos came out very dark (as expected after what you guys already told me) Then I took another look at the flash itself and it has 2 buttons that have a + - on it and under it reads SYNC so I backed it down to 1/8 and I think I got lucky the first adjustment.. I ran off a few more test shots and it's now in SYNC

my plan is to use this on a umbrella or a soft box. The head on the 540 with turn 360 so it might be easier then I think to set that up too.