The Approve and Unapprove buttons will not show unless as User you have your Purchase Ledger Options set to One or Two Stage Approval.
This can be done by following the below steps;
Click on System
Click on Options
Click on Purchase Ledger Options
Click on the drop down text box next to Type under the Remittances header to change from 'No Payment Approval' to either 'One Stage Approval' or 'Two Stage Approval'
Click on the Save button to confirm the changes.