Apple iMac (21.5 and 27-inch, Late 2009) - Manual (Page 40)

40 Chapter 3 Boost Your Memory
8 Insert the new memory modules in the slots with the notches facing right, as shown in
the illustration.
 If you install additional memory modules, insert them in the open slots.
 If you replace the installed memory modules, insert a new memory module in each
of the front slots closest to the display.
9 Press each memory module firmly and evenly into its slot. You’ll hear a slight click
when the memory module is seated correctly.
10 Tuck the tabs into the memory compartment.
®
Memory modules Tuck the tab