Modifying fields should be done in the product front end, as opposed to by the API. Looking into your Sandbox account, I can see field mappings and the view layout appear correctly.
If you are looking to change your field mappings, you can do so via following the below documentation:
Using Field Mappings - http://bullhorn.force.com/pkb/articles/Documentation/Using-Field-Mappings/p
Understanding our Edit Types - http://bullhorn.force.com/pkb/articles/Documentation/Understanding-Bullhorn-Field-Edit-Types/p
Using View Layout - http://bullhorn.force.com/pkb/articles/Documentation/Using-the-View-Layout/p
If you're still having issues on your side, can you please provide a screenshot with an example of the issue you're encountering, and we can look into it further on our side.