Pull down the View menu and see if "View as Table" or "View as List" is selected.
If "View as Table" and you have FileMaker 11, you can add new field and column directly from this layout if you have the file open with a full access password. There's a + sign at top right of your last column heading you can click for this.
If "View as List", you'll need to open Manage | Database | Fields and add the new field definition there. This method for adding a field works no matter what the view so I prefer using this option even when I'm working with a table view. Once you've defined a new field, you can return to your layout, enter layout mode and use the field tool to add your new field to the layout unless a preferences setting has caused FileMaker to already add it for you. If you see that your new field was automatically added, you'll still need to enter layout mode to position it where you want it to go and likely will need to drag the bottom boundary of the body layout part back up to return it to its original size.