Here is my take on commenting; others may correct the misunderstandings I have.
When you have sufficient reputation (50 or more), the choice to comment on someone else's answer will appear as one of the choices below that answer. If you have provided an answer or question, you will be offered the choice to comment on your own question/answer regardless of your reputation level.
It is preferred to gain reputation before doing comments. However, you can submit an answer containing your comment if it provides a useful addition. Be sure to indicate that you are looking forward to the time when you can comment, so that others will know why you gave an answer instead. Also, make sure your comment counts and is worth reading by others.
Gerhard "Ask Me About System Design" Paseman, 2010.09.03
Dear Prof. Bergvelt, I see you figured out how to register for MO. Gerhard is correct, there is a small minimum number of points (either 15 or 50, I forget, it is in the FAQ under "What is reputation") required for leaving comments. As your webpage suggests involvement in algebraic geometry, it should not be long before your answers give you that.
Until then, any answer that says "This is intended as a comment on ___, but I lack the MO reputation to leave comments" is likely to work. I also leave answers if they are simply too long for a comment box, which has an upper bound of some 500 characters typed.
Well, you are welcome to email me with questions, check on http://www.ams.org/cml for an address. I enjoy email.