When you click on the title of a thread you want to reply to (when you are signed into the site), a reply box will appear at the bottom, after all of the other responses. It will say "post a comment".
You simply type your reply into the big box under the word "comment" and then click on the green "post a comment" button. The "To" box right above the big "comment" box is for you to enter another member's name, to direct your reply to. For example, if you wanted to direct a reply to me, put your cursor in that box and type an "n". Anyone's id who replied to the post, with a username beginning with an "n" will pop up to select from. You would just click on that member id name, and it will then appear in the box. Or you can type in the box yourself. For instance, you could type "ALL", or you can leave that box empty all together.
Try to reply to this and see what happens. If you're still having trouble, you can use the "contact us" feature to contact MedHelp:
http://www.medhelp.org/contactus.htm
Hope this helped!