I wouldn't bother doing it on the controller interface itself. I just add them in like any other pixel and create a separate element for it 1 pixel in size. I then don't assign any color to it through the entire sequence. So in LSP, assign the dead pixel channels 1,2,3 and then start your string from channel 4,5,6...... Likewise if you are putting in a dead pixel half way through your string, assign it for instance 49,50,51 and then start your other element at 52,53,54.....
If your string is 100 and you have 2 null pixels then that means there are 102 pixels connected to that output which 2 of those you will use as null pixels.
in the null pixel setting set the start pixel to be one and then set the pixel count to be 2 because you want only 2 pixels to be null from what i can gather not 100 from what you have put in.
As a suggestion (and a general rule of thumb for these forums), it'd be good if you posted an "after" screenshot, and explain what you changed to get it working.
This not only helps you with your own troubleshooting skills, it'll help anyone else that ends up on this thread.