** This issue is addressed in ArcGIS 9.0 Service Pack 2 (SP2) **
The Label tool crashes when being used on a Maplex labeled layer.
This is a known issue.
FindTextElement attempts to get the generated unplaced labels property from the LabelEngineLayerProperties by calling get_BasicOverposterProperties. This pointer is null in the Maplex properties and it crashes when attempting to access the property of a null pointer.
** This issue is addressed in ArcGIS 9.0 Service Pack 2 (SP2).
Click on the link in Related Information below and download Service Pack 2, if necessary. **