I've added a new attribute. It's setup exactly the same way another attribute is that works fine. It doesn't show up as an option when I add a new configurable product. I've refreshed and cleared the cache.
If you want to use it as an option for configurable product you must choose it when you initially create the product. You can't just add it later (unless you want to change database records manually). I recommend that you just create new configurable product. BTW, make sure your new attribute is in chosen attribute set first.