WARNING: This page is about a different car, the 2010 Nissan Sentra. However, it is still accessible from the selected car via links, so may be relevant.
MR20DE (2.0L)
Fig 1: Locating Engine Serial Number - MR20DE (2.0L)
Courtesy of NISSAN MOTOR CO., U.S.A.
QR25DE (2.5L)
Fig 2: Locating Engine Serial Number - QR25DE (2.5L)