Okay, what are you trying to do? If you want to lock the field when an
entry exists in that field, you'd apply your code in the current event and
the Change or Update event of the particular field.
If you put it in the Current event, then when you open the form it will look
at the currently displayed record and make the field enabled or not
depending on whether there is an entry in that field. When you press PAGEUP
or PAGEDOWN or use the navigation buttons to move to another record, then it
will enable that field based on whether there is an entry in the currently
displayed field.
Putting it elsewhere in the code would allow you to make an entry in a new
record, once you tab out of the field, it would update the enabled property.