Since Project Navigator does not support remote IP core integration in ISE 9, one of the following two options could be used.
The recommended procedure is to copy the core to the project directory. Do not use "ADD SOURCE" to add a remote XCO to your project. This adds a pointer to the ".xco" file in its existing directory; however, implementation tools (such as NGDBuild) will not be able to locate the associated core files. Use "ADD COPY OF SOURCE" to add a copy of the XCO and all of the files associated with the core into your project directory.
If the core cannot be copied to the project directory, add the core wrapper (<core_name>.v of <core_name>.vhd) to the project instead of the .xco file. Then, with In Translate -> Options, set the "Macro Search Path" to the directory in which the core files are located. This allows the tools to source the files. However, Project Navigator will not directly interact with the core's .xco file in the event that changes need to be made to the core. Changes can be made using CORE Generator as a stand alone tool.