Contrary to what used to happen with Firewire, most cameras cannot even stream the contents of the viewfinder over usb, so that would be the first thing to check - can the camera stream video over usb?
Even if the camera does support it, it almost certainly won't work with FCP X, as you were already told.
FCP X requires timecode, so even with a Firewire camera you would not be able to record directly from the viewfinder into FCP X.
As Alchroma already said, Quicktime Player X or the Pro version of old Quicktime 7 *might* work.