New editable Drop Down box for Approved Vendors form

Hello,

We have a requirement to add a new field(drop down) to approved vendors form. I was able to add a field to the table and form. But that field is always read-only. I checked AllowEdit property and Enabled property and everything looks correct. I noticed other 2 editable fields on this form use DataMethod, instead of data source. Could you please let me know how to make my new field editable of how to write a edit method and bind via Data method?

Regards,

Dumindu