PlanAhead does not support fnf files, and the modular design flow as well has been been largely replaced by partitions. Although I'm not as familiar with the floorplanner, I believe the equivalent is the UCF file. FNF is floorplanner-specific and is a netlist/constraints file.
Can you please explain exactly what you are trying to do? The modular design flow is tightly tied to older versions of the ISE tools and PlanAhead, and is not supported in 11.
Greg Daughtry Vivado Product Marketing Director, Xilinx, Inc.