Hi,
You did not say which version of ISE you were using but I have had it working for version 4.2, 5.1 & 6.3.
I have a few things for you to check: (1) Have you created an Implementation constraints file (.UCF) and is it associated with the top level of your design? It should appear as a "U" module immediately below (and indented) your top levels "V" entry in the "Sources in Project" list. If not add new source of type "implementation constraints file".......
(2) Uses the Edit Constraints(text) option in the processes list (in the "User Constraints section) you should see some lines like...
NET "ssg" LOC = "E14" ; NET "ssg" LOC = "G13" ; NET "ssg" LOC = "N15" ; NET "ssg" LOC = "P15" ; NET "ssg" LOC = "R16" ; NET "ssg" LOC = "F13" ; NET "ssg" LOC = "N16" ; NET "ssg" LOC = "P16" ;
where "ssg" is the name of the signal in your top level entity declaration. You can put the lines in the UCF manually, the definitions are in the examples on Digilent's web site.
Hope this helps,
Tim