Do you have a value list defined that uses the specify field option to list the values from this table?
Is the code field of type "number" or "text"? It looks like all you need to do is convert the code field to text in order for this to work. If this table of values were imported into FileMaker this would be simple to do. If you are linking to an external Excel file, I'm not sure how to get that column in your excel sheet to be identified as "text" instead of number.
I will review my entries.