The comment form is turned on / off in the regular WordPress way.
In your Admin Panel go to Settings->Discussion and check the settings.
It may turn out that the comments are disabled (“Allow people to post comments on new articles” unchecked). You also may be missing the comment form because you’ve allowed commenting only to the registered users (“Users must be registered and logged in to comment” checked) or because the comments are being closed automatically (“Automatically close comments on articles older than…” checked). Here are example settings from the demo that always show the commenting form:
You may also have the comments disabled for each post/page individually. In this case you need to enable them individually.