Apple PowerBook (Memory (RAM) Replacement) - Manual (Page 11)

PowerBook (FireWire) Memory (RAM) - 11
7 Press down very firmly against the processor module until you feel it
securely attach to the connector on the logic board. Then replace the
screw.
Note: If you have a 2-inch RAM card in the upper slot, remove it
before you reinsert the processor module; the card blocks the area
you need to push against to secure the processor module in place.
Note: If the processor module is not securely connected to the logic
board, your computer will not start up.
8 Go to “Closing the Computer” to complete the reassembly process.
Screw
Processor
Module
RAM Card