How do I create a 5-digit slider expression? (It's for a lab test LCD screen animation.)

Here's a cleaner way to achieve what you are after:

function pad(string, length, character) {
	// Make sure string is String
	string = (typeof string === 'string' || string instanceof String) ? string : string.toString();

	// If not defined, set padding length to 2
	length = length || 2;

	// If not defined, set padding character to '0'
	character = character || '0';

	while (string.length < length) {
		string = character + string;
	}

	return string;
}

var value = effect("Slider Control")("Slider").value;

// Round value to 1 decimal place
var valueAsString = value.toFixed(1);

// Pad string to total of 6 characters
var paddedString = pad(valueAsString, 6, '0');

// Add customn prefix
var result = '+0' + paddedString;

result;

Not elegant, but you can follow the same logic already applied:

sliderRaw = effect("Slider Control")("Slider").value.toFixed(1);

if (sliderRaw < 10){
slider = "+0000"+ sliderRaw;}

if (sliderRaw >= 10 && sliderRaw < 100){
slider = "+000"+ sliderRaw;}

if (sliderRaw >= 100 && sliderRaw < 1000){
slider = "+00"+ sliderRaw;}

if (sliderRaw >= 1000 && sliderRaw < 10000){
slider = "+0"+ sliderRaw;}

if (sliderRaw >= 10000 && sliderRaw < 100000){
slider = "+"+ sliderRaw;}

slider;

*Martin