You're almost there... A comma is not a valid end-of-line character in JS. A semi-colon is. And you have to make sure you use only standard double-quotes, not any "fancy" ones, for your code to work. It might be a good idea to read a tutorial about the core JS syntax, like this one: http://www.w3schools.com/js/default.asp
Here's a fixed version of your code:
if (event.target.value!="Off") { this.getField("Check Box1-1").readonly = true; this.getField("Check Box1").readonly = true; } else { this.getField("Check Box1-1").readonly = false; this.getField("Check Box1").readonly = false; }