messages[4] = "FIFTH MESSAGE"
in the top of the script, just change the numbers appropriately. Don't forget to change the messages!
document.write('<INPUT TYPE="button" VALUE="start" onClick="startSign()">')
Now put this inside your body code.
<body onLoad="startSign()">
<BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#FF0000" VLINK="#000080" ALINK="#000080" onLoad="startSign()">
<body onLoad="startSign()">