Hi
I think this will help.
Double-click the field to get into its properties. Then click the fx button on the end of the default value row. Select 'Insert Field or Group' and browse to your table and the field you want.
At this point there is an option to filter the data by a formula or a value on the form, for example, I type in my employee number and the NAME field is populated from the database filtering the options to show the name where the employee number matches the number i typed in.
Either way when you have set things up to your satisfaction OK out of there and test.
Should work a treat?