Click on the link All Documents with an Open Public Comment Period
Locate and click on the link APHIS-2006-0168
Click on "Add Comments" in the line of this item.
You have two options for adding your comments: you can either (1) enter them into the comment box or (2) send them as an attachment. What is not obvious is that even if you send something as an attachment, you must still enter something into the comment box. Enter something like, "See attachment" into the comment box and then click on the Next Step button.
If you have "successfully" submitted a comment, you should receive a confirmation tracking #.
You may want to check the web page again to confirm that your comments actually made it to the list.