How to show & hide line numbers
There's three ways to toggle line numbers:
If you're registered (which is free), then you have full control over the editor preferences, including line numbers. The advantage of this approach is that it'll save against your account so it carries across browsers and machines.
Simply double click on the panel language selector.
Note that this preference will only save in the browser you're using at the time, and doesn't carry across to other browsers or machines.