To access this page click on Ad Activity History while on an Ad's Summary Page.
The Ad Activity History tab lets you see what changes have been made to your Ad since it was created.
On the Ad Activity History tab you can search for keywords and use the Activity Selector to filter by dates, filter by API that was used to make the changes. The Columns drop down menu allows you to choose what information you want to see on the page, including viewing the JSON information from the change request.