If you want to add a new attribute field to your attribute table, open the attribute table, click on the upper left dropdown menu that says "Table Options" and click "Add Field..."
If you want to edit existing fields names, open ArcCatalog, navigate to your data, right click on it, choose "Properties...", click on the fieldname you would like to change and rename it.
If you are talking about adding data to your attribute table, you need to start an editing session by clicking on Editor -> Start Editing.
The Editor Pencil you are talking about disappeared with ArcGIS Desktop 10.