Not sure why you can’t see the respond field. Below the post should be my profile and then three other posts that have been published to Snapshots and then below those you should see:
Responses (then a blank space) Write a response
Click on “Write a response” and you should get the respond field.
Things look different if you are accessing Medium from a browser rather than an app so that might be what is confusing you.
To tag someone, hit the @ key and it will bring up a field where you can start typing the person’s name. When the name appears in the list (you may have to scroll down), click it and the name will appear in your post. Now, I will warn you that I have found tagging people in posts to be glitchy. Sometimes I can find the person in the list and sometimes not. Sometimes it works flawlessly, sometimes not.