Before you can comment on a VoiceThead presentation, you'll need to register for an account under your Maine.edu email, then log in to your account. Also, make sure that you've set up your identity in VoiceThread before you add comments to any presentations.
Watch this video from VoiceThread to learn more about how to add comments to a presentation and the different commenting options that might be available to you.
VoiceThread has different "How to Comment" guides for its website and mobile app versions. They detail everything there is to know about commenting in VoiceThread.