Hopefully you have a free PCI slot available, whwn you take the side off you will see some long white sockets towards the rear.
If there is, carefully remove the blanking plate opposite the slot where you are to fit your firewire card.
Carfully offer up the card and push down gently. Sometimes a little rocking motion side to side, longways, is needed.
You should have a screw supplied with the card or if the blanking plate is not 'tagged' to the chassis, you will have one from there.
Insert the screen. Replace side. Reconnect.