When running an anonymous PL-/SQL Block and coming to a variable, e.g. '&new' the SQL Navigator does not show the appropriate popup for entering the variables value.
There's an option in the toolbar of the Code Editor controlling whether to show the substitution variable dialog.
You can find it next to the Spool button and it looks like ':&', with ToolTips showing 'Scan Defines/Substitutions'.
This option is OFF by default.