Unfortunately many of the Xilinx schematic downloads are not complete with missing symbols and files. This was one of the reasons driving the architecture change Mentor Graphics initiated with the 2007 release of DxDesigner, moving designs from hundreds if not thousands of files into an integrated database. Designs are much easier to share with cached parts and minimal files.
First all the symbols need to exist, and if they represent a part they must be of type module,
Second, complete wir files need to be created for the design, later versions of PADS perform this step automatically. I have attached a command line wir file generator in check.zip Use the readme.txt for instructions.
Even then you may not be successful as many of the downloads are not complete.