How to add a text Autocomplete Widget programmatically
Ok, so if you're trying to do it through hook_form_alter, you have to read the documentation on altering CCK fields. 

The short version: the CCK fields are processed after the form is built. If you want to change anything in them, you have to add a callback to $form['#after_build'] (but do not replace the array, just add to it). Then implement the function, change values in $form and return the $form.