Finding the mean of a set of numbers

You never return a value in Add(), change x = ... to return ...

function Add()
    return First.Value + Second.Value + Third.Value + Fourth.Value + Fifth.Value
end

Also, be careful, if you call Add() before First, Second, etc, are defined, it'll error. It's better to define variables before functions.

Here's a rewritten version:

First = script.findMean.First
Second = script.findMean.Second
Third = script.findMean.Third
Fourth = script.findMean.Fourth
Fifth = script.findMean.Fifth
Plus = script.findMean.Plus

function Add()  
    return First.Value + Second.Value + Third.Value + Fourth.Value + Fifth.Value
end

--

First.Value = (math.random(1,10))
Second.Value = (math.random(1,10))
Third.Value = (math.random(1,10))
Fourth.Value = (math.random(1,10))
Fifth.Value = (math.random(1,10))

--

print(First.Value)
print(Second.Value)
print(Third.Value)
print(Fourth.Value)
print(Fifth.Value)

--

print("-")

Plus.Value = Add()
print(Plus.Value)

print("-")

print(Plus.Value / 5)

By the way, this really isn't a good way to do it. It's better to use a table rather than use a bunch of NumberValues.

Here's a simple way to get the average of numbers in a table:

local numbers = {23,5,2,12,10}

function average(table)
    local sum = 0
    for i = 1, #table do
        sum = sum + table[i]
    end
    return sum/#table
end

average(numbers) --> 10.4

The only main problem I can see, is that the function is above your variables, and when you are trying to access your variables, it can't, because that part of the script hasn't been loaded yet.

Try putting the function below your variables.