To use OS Lion, you need to use the latest Beta H8000 software (see H8000 support section for more info).

Assuming you are doing this, and have studied the section "Configuring the H8000FW for
Macintosh Output" in the User Manual your problem may be in understanding the H8000's routing arrangements. Probably the easiest way to get a result is to use one of the Factory Routings, e.g. "FIREWIRE A || B" on p.32 of the UM.

Another option would be to play with the Routing Utility – see H8000 support section for more info.

Don't know anything about DP, so can't be any use if that is where your problem lies.