In ISE 9.1i, Project Navigator does not properly handle the support files for IP cores with EDN netlist.
As a method by which to work around this issue, if the IP core supports VHDL, the Functional Model Target Language for the core can be changed to VHDL. To do this, select the IP core in the sources window, then right-click View HDL Functional Model, and select Properties.
This problem has been fixed in the latest 9.1i Service Pack available at:http://www.xilinx.com/xlnx/xil_sw_updates_home.jsp
The first service pack containing the fix is 9.1i Service Pack 1.