Dell XPS 15 (L502X, Early 2011) - Manual (Page 19)

Memory Module(s) 19
3 Slide the memory module firmly into the memory-module connector at a
45-degree angle, and press the memory module down until it clicks into
place. If you do not hear the click, remove the memory module and
reinstall it.
NOTE: If the memory module is not installed properly, the computer may not
boot.
4 Replace the module cover (see "Replacing the Module Cover" on page 16).
5 Replace the battery (see "Replacing the Battery" on page 14).
6 Connect the AC adapter to your computer and an electrical outlet.
7 Turn on the computer.
As the computer boots, it detects the additional memory and automatically
updates the system configuration information.
To confirm the amount of memory installed in the computer:
1 tab 2 notch
3 memory-module connector
3
1
2