Go to Setup > Build> Create> Workflow & Approvals>Approval Processes> Select the name of the approval process for which you wish to add fields > Click on Edit > Approval Process PageLayout > Add fields and Save
aah...you meant the comment inputfield on the approval page - unfortunately that is something yet to be implmented in Salesforce: An idea does exist on our Idea exchange for the same : https://success.salesforce.com/ideaView?id=08730000000BpKk
Hope this helps!
we have one comments input field in approval page layout but I want one more inputfield in approval pagelayout ? telme how to add it ?