Xilinx Xpower Analyzer: Expected scope definition in VCD
I don't know exactly what is going wrong, but these are the two lines I use in a modelsim '.do' script to get a .vcd file. The '.do' file is just a tcl script, so I assume you can adapt it quite easily or maybe even use it directly in Xpower.

vcd file output.vcd
# add all the appropriate signals (only inputs) to the vcd, -r, recursively
vcd add -in -r *