When I attempt to create an OFFSET IN constraint from the Advanced tab for the Pad-to-Setup dialogue window, no registered time groups are available; I have created several PERIOD constraints, but none of them appear.
This problem has been fixed in the latest 6.1i Service Pack, available at:
The first service pack containing the fix is 6.1i Service Pack 2.
You can also work around this problem by typing the registered group into the OFFSET constraint by hand using a text editor.
Another way to work around this issue is to create a time group based upon INST TNM groups, which will appear in the registered time group listing of the OFFSET IN dialog window.