1) Email your adviser.
2) Get an add/drop form just outside of the place where you turned in your registration form.
3) Fill out to drop 8.01 and add 8.01L (you can do this on the same form).
4) Go to whatever 8.01L section fits your schedule and ask to add the class. If the prof is cool with it (highly likely), the prof will sign the form.
5) When your adviser returns, have her/him sign the form. He keeps a copy, you keep a copy, and you turn in a copy.
You can add classes until October 9th, so don't worry too much. (You can drop classes until November 18th with nothing showing up on your record, so don't worry about that either.)
I believe that if you've already turned in your registration form, you can't modify it before the first day of classes anyway. I'm not certain about that... but yeah, don't worry, it doesn't matter either way