Why wont this player added thing work?

Your issue seems to be an obscure one which not even I knew about until now; when a player first enters (at least in Play Solo; not entirely sure), their name is set to Player1 before being set to the correct username. I found that simply adding a wait() fixed this.

game.Players.PlayerAdded:connect(function(plr)
    wait()
    if plr.Name == "jaxxcoolboy" then
        script.Parent.Text = "The founder jaxxcoolboy is in the game!"
    end
end)

No on PlayerAdded you don't want a variable so instead of

local plr = game.Players.PlayerAdded:connect(function(plr)

you'll want to do this

game.Players.PlayerAdded:connect(function(plr)

a function cannot be a variable if you want it to run. or you could do this

function onPlayerEntered()
--code
end

game.Players.PlayerAdded:connect(onPlayerEntered)

And always remember to put an end) if you're doing the first one