If you have both ISE/ChipScope 7.1 and ISE/ChipScope 8.1 installed, after installing 8.1 you will no longer be able to add a ".cdc" file to a 7.1 project.
To work around this issue:
1. Install ChipScope 8.1i. This will set up the registry for ChipScope 8.1i.
2. Force registry to point at 7.1i ChipScope Installation. This may be done in two ways:
a. Reinstall ChipScope 7.1i, or
b. Change the registry entry HKEY_LOCAL_MACHINE/SOFTWARE/Xilinx/ChipScope/CURRENTVERSION to 7.1 PRO manually with regedit.
3. Launch ISE 8.1i. Manually point ISE 8.1i at the ChipScope 8.1i installation. Edit->Preferences->Integrated Tools. In the ChipScope box, pick the location containing the ChipScope 8.1i bin/nt directory.