Unable to select Undo Advisor because it is grayed out.
The Undo Advisor is only available in the commercial version of Toad for Oracle with the optional Quest DBA Module. Also, the Undo Advisor only works with Oracle versions10g or higher.
In versions Toad 10 and above, unless you have instructed Toad to make this window accessible, this functionality is disabled. You can restore functionality from View | Toad Options | Windows and clicking the Available checkbox in the appropriate row of the grid.