The example posted in your OTHER thread showed how to do it. Did you try that example?
Where (Somefield = Forms!yourform!yourcbo OR Forms!yourform!yourcbo Is Null)
If there is a value selected, Somefield must be = that value OR if the form field is null, then the record is selected. So - pick yes, no, or NOTHING. If you pick yes/no the value must match. If you pick nothing, then that part of the condition will be true. and so it doesn't matter if Somefield is equal to the selection.