Hi -
You have to split the backplane just like in the Nx5500 series. The split is done on a per-slot basis. A quick read of the Interface Configuration Guide reveals that the 56128 only supports native FC on expansion modules in slots 2 and 3.
Nexus 5600 Interface Configuration Guide
Additionally, the guide implies that the configuration is backwards from other Nexus switches in that FC must be configured starting at port 1.
Assuming that you are configuring Module 2, the setup would go like this:
slot 2
port 1-24 type fc
exit
copy run start
poweroff module 2
no poweroff module 2
Once the module comes back online (3-5 minutes) you will be able to start your native FC configuration.
PSC