Selecting an ID field in lookup wizard

Hi
When the user creates a drop down field in the tutorial at http://xlinesoft.com/tutorials/dependentdropdowns.htm, he creates a look up field where the 'link field' an id field.
When I want to create a look up field and select a link field, I can't choose any id field because they are 'greyed out'. It doesn't matter whatever table I select. What should I do that I can select an Id field here?
Yes, I am a newbie. Thanks so much for getting me out of that situation.