From the media library, you can insert one or more clips by selecting them (using the Ctrl or Shift keys to select multiple clips) and:
Dragging and dropping them onto place in the timeline.
Using Library ▸ Insert at End of Timeline.
Pressing the Insert (Ins) key. Those clips will be inserted at the end of the timeline.
It should also be possible to import files directly into the timeline by dragging them from another application and dropping them onto PiTiVi's timeline. The files would be added automatically to the media library in the process. This feature is not yet implemented, see bug #570118 for more information.