As Xilinx is no longer supporting those boards, you have to install the board support for them in your XPS environment.
Check out AR35623 that describes the necessary steps to set up and get XPS to work with boards such as ML501!
Create a directory structure similar to the following:
C:\ < Global_Repository > \ < Library_Name > \boards
- Download the zip file containing the ISE software 11.5 boards which are now "Legacy Boards"and unzip itto the newly created boards directory.
- Set the Global Repository setting in XPS through the Edit > Preferences > Application and fill in your Global Repository directory name in the Global Repository Search Path data entry field.
- Restart XPS running Base System Builder. The now old board file will be available for BSB.
Hope this helps.