Apple PC Compatibility Card (Installation) - Manual (Page 13)

13 Installing the PC Compatibility Card in a 5400 Series Macintosh Computer
3 Remove the plastic access port cover from the vertical plate.
Squeeze together the two plastic tabs holding the access port cover in place;
then pull the access port cover off from the outside.
Plastic access port cover
Vertical plate
PCI slot
Metal retainer
Vertical plate
Squeeze the two plastic tabs together.