The Append and Add Child commands are enabled when a node in Grid View is selected. The Append command adds a new element node as the last sibling of the selected item. The Add Child command appends a new element node as a child. Change the name of the newly added element by double-clicking in its name cell and editing. Change the node type by clicking the element's icon (to the left of its name) and selecting the node type you want, or by using the command XML | Type.
See the topic XML | Document Structure for more information.