Asus M2N-LR/SATA - Manual (Page 29)

2-0 Chapter 2: Hardware information
2. Attach one end of the retention bracket
to the retention module base.
3. Align the other end of the retention
bracket (near the retention bracket
lock) to the retention module base.
A clicking sound denotes that the
retention bracket is in place.
4. Push down the retention bracket lock
on the retention mechanism to secure
the heatsink and fan to the module
base.
Make sure that the fan and
heatsink assembly perfectly fits
the retention mechanism module
base, otherwise you cannot snap
the retention bracket in place.