I usually just put in the FEIN.  If there's state withholding, there really should be a state ID number.  Usually the problem is that there's no state W/H but the software is still inexplicably grumpy so it probably doesn't matter what you put in there to make the error go away.  My concern when there's state withholding but clearly the employer is not competent enough to include the ID number, are they competent enough to be filing W-2s with the state and remitting the taxes withheld?