Add Element
To insert a new element at the current cursor position or - in the case if something is selected - to surround the selected text or fragment with a new element:

Choose Document | Add | Element... (or hit Ctrl+Space)
In the content assist which pops up choose the element to add by one of the following possibilities:
Use Up and Down keys to select element and hit Return
Click on the element to add
To filter the list of elements type the name or a part of the name and hit Return to add the topmost element
Tip: To place the cursor at the proper position (e.g. a new chapter between two existing chapters in DocBook) see the path which is shown in the status bar.