In my VHDL flow, first time I run the DCM Wizard, the DSSEN pin is grounded correctly. However, if I re-open that existing XAW file in the DCM Wizard and save it again, the DSSEN pin is no longer grounded. VHDL synthesis will then fail because the DSSEN pin is floating.
This problem is fixed in the latest 5.1i Service Pack, available at:
The first service pack containing the fix is 5.1i Service Pack 2.
Alternately, you may work around the problem by manually connecting DSSEN to GND after generating the VHDL file.