You never set a command for the button. On the GUI creator screen, there's a field called "Command" you fill that in with a command, for example:
testbutton();
That could be your command and then you would put this in the client.cs:
function testbutton()
{
messageboxOk("Test","Thanks for pushing the test GUI button!");
}
Or something like that. Re-read the creating part of the Tutorial, I haven't read through this in a while.