If you select Check For New Comments and someone added a Comment you should see a blue or red dot appearing in the sidebar of iWeb. You would then select the PhotoPage or the Blog-Entries page to see in detail to which Entry the comment was added to (again a blue dot should display near it).
From iWeb Help about deleting (thus also viewing) Comments
To delete a comment and its attachment:
1. Select the comment to delete:
◼ To delete a blog comment, select the Entries page in the sidebar (on the left side of the iWeb window), and then select the entry in the Title list (at the top) that contains the comment.
◼ To delete a comment on a photo, select the Photos page in the sidebar that contains the photo, and double-click the photo to go to its detail page.
2. Click the Delete button (it looks like an X) next to the comment.
If you are connected to the Internet, the comment is deleted immediately from the website (you don’t have to republish).
Unfortunately I don't use MobileMe (.Mac) myself...