Samsung ML-2251NP - Manual (Page 181)

I NSTALLING P RINTER O PTIONS
E.4
4 Locate the memory DIMM slot (the upper slot) on the
control board. The PS DIMM is installed in the lower slot.
Slots are not interchangeable.
5 Hold the memory DIMM so that the notch and the
connection point are aligned with the slot as shown, and
insert it all the way into the upper slot.
6 Close the control board cover.
Memory
DIMM
PS DIMM