In View menu, make sure the ‘video playback’ is set to ‘apple firewire PAL'(or whatever you are using)
From same menu, ‘refresh AV devices’ especially if you turn the camera on & off or re-connect it
From same menu, External video set to ‘all frames’
this sets the output so you can play direct from the timeline (after rendering) You should see the video on the camera’s LCD screen even before you record.
To do an assemble edit you need a bit of timecode on the tape to start things off and you need to check under ‘Final Cut Pro’ menu, Audio/video settings, A/V devices that the different settings for output to tape in the bottom half are not ticked (assuming you set playback as above)
Its unlikely but also try another firewire cable