You will need to "Lock" the budget structure in order to make any changes such as adding new budget lines. Locking the structure while editing it precludes someone else from entering data into a budget line that you are editing, thus inadvertently deleting their data.

When locked, the screen becomes read-only to other users who may open that screen. When you have finished your changes, you can either click the Lock icon to unlock the structure or just close the program.