To seek within the currently playing media item drag the stylus across the playback position slider above the playlist area. As you move your stylus across the slider the playback time will update to indicate the value of the new position. When you release the stylus playback will jump to the new position. If you decide that you do not want to change the song's current position drag the stylus off of the playback position bar before releasing it.
Another method of seeking is to set a hardware button to the fast-forward or rewind functions via the settings screen. Once set you can begin fast-forward or rewind playback by pressing the mapped button, which are equivalent to activating the media player's fast-forward or rewind functions.