Someone asked this yesterday so thought I'd share the answer in case others are wondering the same.
If you receive these posts as emails direct to your Inbox, you first need to click the title of the post in the email so that you are taken to the blog itself.
Then, at the bottom of the post, you will see a link that says "No comments" (or "2 comments" or whatever the number is). Click it and a comment box will open up for you.
If you're still having trouble, email me at email@example.com and I'll help you :)