I haven\t done any of this programming on arduino but when I look at the code the problem I think you have is that when you type the exact URL for a button in the web browser the web browser (as it should) sends a HTTP GET message. The buttons on the page makes the browser send a HTTP POST message (as it should). The code for switching the light is only run when a HTTP POST message is received.
If you don't understand this about GET and POST I suggest you google for some HTTP protocol description. And if you do, see if the code for the GET message can retrieve the same parameters as in the POST message. So if there are no parameters in the URL it displays the page, if there is any (valid) parameters it executes the requested function.