Undo/Redo action is located on the quick access toolbar in Microsoft Office 2007, but can we find it on the Ribbon Tab? If yes, at where?
By default they're not in the Ribbon Tab but in the Quick Access Toolbar, Top Left, next to the Save button. Note that the Redo button is only activated after an Undo.
If what you're trying to do is customize the Ribbon, Microsoft's answer is:
The Ribbon, which is part of the Microsoft Office Fluent user interface, is designed to help you quickly find the commands that you need to complete a task. Commands are organized in logical groups that are collected together under tabs. Because each tab relates to a type of activity, such as writing or laying out a page, it is not possible to customize the Ribbon without using XML and programming code.
There is an official Microsoft page showing you how to do it.