51 13... Removing and installing/replacing cover in engine compartment on left or right

Left side: 

Turn locks (1) 90° counterclockwise.

Unlock catch (2).

Remove cover (3).

Fig 1: Identifying Engine Compartment Cover And Catch

Courtesy of BMW OF NORTH AMERICA, INC.

Right side: 

Turn locks (1) 90° counterclockwise.

Unlock catch (3).

Remove cover (2).

Fig 2: Identifying Engine Compartment Cover, Catch And Lock

Courtesy of BMW OF NORTH AMERICA, INC.