When creating labels with the Label Editor, you may notice that you can open up the Label Editor for some tables and not for others. Tables that fail to open in the Label Editor are probably set to "Invisible." So for those tables you wish to use with the Label Editor but cannot, check your settings under the Triggers tab it is most likely that the table properties is set to "Invisible". If you uncheck this setting you will notice that this table can then use the Label Editor.

Other places where invisible tables cannot be used are:

All query editors
Order By editor
Chart Editor
Import/Export dialog boxes
Apply Formula dialog box
For more information see "Setting Table Properties" in Chapter 2 of the 4th Dimension Design Reference.