Players do not have a folder in them called 'StarterPlayer'. What you are looking for is called 'StarterGear'.
'StarterPlayer' is a service, a child of 'game'.
http://wiki.roblox.com/index.php/Place_hierarchy
EDIT:
You won't be able to do it properly, your hierarchy needs to be this:
- StarterPlayer
- StarterPlayerScripts
- Script (StartTheGame) (this puts the script into a player when they join)
Your script should be:
1 | pl = script.Parent.Parent.Parent.Parent |
2 | script.Parent.MouseButton 1 Click:connect( function () |
3 | pl.StartTheGame.Disabled = false |