When I push the key down, the Gui wont appear?

UPDATED

local Player = game.Players.LocalPlayer
local Mouse = Player:GetMouse()
local Gui = game.StarterGui
local DevMenu = Gui.DevMenu
local Main = Gui.DevMenu.MainFrame
local Sound = DevMenu.Activater.Sound_1

if Player.Name == "Player1" then
game:GetService("UserInputService").InputBegan:Connect(function(input,chat)
if not chat then
    if input.KeyCode == Enum.KeyCode.X then
		Sound:Play()
		print("X is down.")
        DevMenu.Enabled = true
        else
        DevMenu.Enabled = false
            end
        end
    end)
end

For some reason this wont make the Gui appear when I press x. This is in a LocalScript. When I press X the code fires and I see the the "X is down" in output. Any help is appreciated!

You should be using PlayerGui, not StarterGui. PlayerGui is where GUI's get cloned from the StarterGui to a player.

local Player = game.Players.LocalPlayer
local Mouse = Player:GetMouse()
local Gui = Player.PlayerGui
local Main = Gui.DevMenu.MainFrame

if Player.Name == "AdvancedCode" then
game:GetService("UserInputService").InputBegan:Connect(function(input,chat)
if not chat then
    if input.KeyCode == Enum.KeyCode.X then
        Main.Visible = true
        else
        Main.Visible = false
            end
        end
    end)
end

Ok, So you need to know how StarterGui works. Apparently StarterGui is where you place your Guis correct? Follow Skeletal's instructions first please so it will work right. So, you need to use PlayerGui instead because your just editing something that's not on your screen. XD

Gui = Player.PlayerGui

That should help!