Sorry, that feature isn't available then...
From FileMaker's help here ( http://www.filemaker.com/12help/html/edit_layout.10.30.html )
Auto-complete is not supported in drop-down lists when the value list used is configured to use Also display values from second field, Show values only from second field, or Sort by second field. For more information, see Defining a value list for data entry.
hope that helps...
Court Bowman, CEO
Cleveland Consulting, Inc.
Visit us on the web at http://www.clevelandconsulting.com