Apple Mac mini (Mid 2011) - Manual (Page 45)

45 Chapter 3 Boost Your Memory
Step 6: Replace the bottom cover.
Replace the bottom cover, using the alignment dots to place it in the unlocked position.
Align
the dots
Rotate the cover clockwise to lock it in place.
Locked
WARNING: Don’t connect the power cord or turn on your Mac mini until you replace
the bottom cover. Never operate your Mac mini without the cover in place.