@cocasio The Invoice header fields contain one field labeled Comments. This is where the comment can be entered. I would also recommend that the person adding the comment send an email to let the approver know they added a comment. Also, the trick is whether or not the approver knows where to go to view the comments. When viewing the Invoice, use the Details link to view all comments.
I hope this helps.
@cocasio The system doesn't allow for adding comments directly to the History. I looked at your site configuration and it was setup to not use the Comment field. Your company is using Description instead, but I do not believe entering a description will populate the Comment history based on what I saw in your configuration. You will need to reach out to your site administrator and ask about getting the Comment field added.