The default setting on "Allow Comments" is "Closed." I read some of the documentation for Movable Type, and "Closed" means that you can read the existing comments but not post new ones. After saving this post, I'm going to log out of UThink and see if I'm able to leave a comment.
(Edited to add: It wouldn't let me, so it works. I came back in and changed the setting for just this post to "Open," so you can comment on it now.
Also, if you have posts up, especially those of you who just posted in the last day or two, and would like to allow comments, you can log in to UThink and click "Edit Entries." Go to your post(s) and change the settings as you see fit. Be aware, though, that as of yet I haven't been able to figure out how to arrange it so that only students in our class can comment -- or if that's even possible. I've left a voice mail with The Movable Type Guru, so more on that soon.)
Okay, notice that when you're creating a post, you see a section with four drop-down menus:
Post Status (?)
Allow Comments: (?)
Allow Pings (?)
Text Formatting: (?)
For this post, I selected "None" under "Allow Comments," and as you can see, you can't leave a comment on this post.