Thanks nab. I don't really know much about scripting but after fiddling for a while I worked out the values I had to change.
My alterations are in bold. (Please note this isn't the entire script, just the relevant section)
// main script
// build the palette
var shiftLayersPalette = new Window("palette", "Stagger Selected Layers");
shiftLayersPalette.bounds = [400,300,605,445];
var topPanel= shiftLayersPalette.add("panel", [5,5,200,65], "Stagger Selected Layers");
var currentRB = topPanel.add("radiobutton", [10, 15, 190, 35], "Stagger from current time marker.");
var originalRB = topPanel.add("radiobutton", [10, 35, 190, 55], "Stagger from original in-points.");
var bottomPanel = shiftLayersPalette.add("panel", [5,64,200,115], "");
var framesRB = bottomPanel.add("radiobutton", [10, 5, 190, 25], "Stagger time in frames.");
var secondsRB = bottomPanel.add("radiobutton", [10, 25, 190, 45], "Stagger time in seconds.");
var staggerInput = 1;
currentRB.value = true;
framesRB.value = true;
framesRB.onClick = on_framesButton_click;
secondsRB.onClick = on_secondsButton_click;
var text_input = shiftLayersPalette.add("edittext", [10, 120, 45, 140], staggerInput);
var offsetType = shiftLayersPalette.add("statictext", [50, 124, 90, 145], "frames");
text_input.onChange = on_textInput_changed;
var aboutButton = shiftLayersPalette.add("button", [90,120,120,140], "?");
var shiftButton = shiftLayersPalette.add("button", [127,120,190,140],"Stagger");
aboutButton.onClick = on_aboutButton_click;
shiftButton.onClick = mainShiftFunction;
// display the palette.
Now it works a treat on Windows. All credit to Paul T for creating it in the first place. It's very handy.